New sub, gsub, and ssub verbs

[johnkerl]

For issue #1356

Examples:

$ mlr --icsv --opprint --from docs/src/example.csv cat --filename
filename             color  shape    flag  k  index quantity rate
docs/src/example.csv yellow triangle true  1  11    43.6498  9.8870
docs/src/example.csv red    square   true  2  15    79.2778  0.0130
docs/src/example.csv red    circle   true  3  16    13.8103  2.9010
docs/src/example.csv red    square   false 4  48    77.5542  7.4670
docs/src/example.csv purple triangle false 5  51    81.2290  8.5910
docs/src/example.csv red    square   false 6  64    77.1991  9.5310
docs/src/example.csv purple triangle false 7  65    80.1405  5.8240
docs/src/example.csv yellow circle   true  8  73    63.9785  4.2370
docs/src/example.csv yellow circle   true  9  87    63.5058  8.3350
docs/src/example.csv purple square   false 10 91    72.3735  8.2430

$ mlr --icsv --opprint --from docs/src/example.csv cat --filename then ssub -f filename .csv ""
filename         color  shape    flag  k  index quantity rate
docs/src/example yellow triangle true  1  11    43.6498  9.8870
docs/src/example red    square   true  2  15    79.2778  0.0130
docs/src/example red    circle   true  3  16    13.8103  2.9010
docs/src/example red    square   false 4  48    77.5542  7.4670
docs/src/example purple triangle false 5  51    81.2290  8.5910
docs/src/example red    square   false 6  64    77.1991  9.5310
docs/src/example purple triangle false 7  65    80.1405  5.8240
docs/src/example yellow circle   true  8  73    63.9785  4.2370
docs/src/example yellow circle   true  9  87    63.5058  8.3350
docs/src/example purple square   false 10 91    72.3735  8.2430

$ mlr --icsv --opprint --from docs/src/example.csv cat --filename then sub -f filename '.*/' ''
filename    color  shape    flag  k  index quantity rate
example.csv yellow triangle true  1  11    43.6498  9.8870
example.csv red    square   true  2  15    79.2778  0.0130
example.csv red    circle   true  3  16    13.8103  2.9010
example.csv red    square   false 4  48    77.5542  7.4670
example.csv purple triangle false 5  51    81.2290  8.5910
example.csv red    square   false 6  64    77.1991  9.5310
example.csv purple triangle false 7  65    80.1405  5.8240
example.csv yellow circle   true  8  73    63.9785  4.2370
example.csv yellow circle   true  9  87    63.5058  8.3350
example.csv purple square   false 10 91    72.3735  8.2430

$ mlr --icsv --opprint --from docs/src/example.csv cat --filename then sub -f color,shape l X
filename             color  shape    flag  k  index quantity rate
docs/src/example.csv yeXlow triangXe true  1  11    43.6498  9.8870
docs/src/example.csv red    square   true  2  15    79.2778  0.0130
docs/src/example.csv red    circXe   true  3  16    13.8103  2.9010
docs/src/example.csv red    square   false 4  48    77.5542  7.4670
docs/src/example.csv purpXe triangXe false 5  51    81.2290  8.5910
docs/src/example.csv red    square   false 6  64    77.1991  9.5310
docs/src/example.csv purpXe triangXe false 7  65    80.1405  5.8240
docs/src/example.csv yeXlow circXe   true  8  73    63.9785  4.2370
docs/src/example.csv yeXlow circXe   true  9  87    63.5058  8.3350
docs/src/example.csv purpXe square   false 10 91    72.3735  8.2430

$ mlr --icsv --opprint --from docs/src/example.csv cat --filename then gsub -f color,shape l X
filename             color  shape    flag  k  index quantity rate
docs/src/example.csv yeXXow triangXe true  1  11    43.6498  9.8870
docs/src/example.csv red    square   true  2  15    79.2778  0.0130
docs/src/example.csv red    circXe   true  3  16    13.8103  2.9010
docs/src/example.csv red    square   false 4  48    77.5542  7.4670
docs/src/example.csv purpXe triangXe false 5  51    81.2290  8.5910
docs/src/example.csv red    square   false 6  64    77.1991  9.5310
docs/src/example.csv purpXe triangXe false 7  65    80.1405  5.8240
docs/src/example.csv yeXXow circXe   true  8  73    63.9785  4.2370
docs/src/example.csv yeXXow circXe   true  9  87    63.5058  8.3350
docs/src/example.csv purpXe square   false 10 91    72.3735  8.2430