[feat] Multiple template sources.

[lukesneeringer]

This commit allows for multiple sources for templates, which are searched in order.

[lukesneeringer]

Also adds documentation covering both local and Docker-based invocation. No change to gapic.sh at this time since I am not sure I want to add the directory mounting aspect to that.

[codecov]

Codecov Report

Merging #88 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #88   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          15     16    +1     
  Lines         710    733   +23     
  Branches      144    150    +6     
=====================================
+ Hits          710    733   +23

Impacted Files	Coverage Δ
gapic/generator/options.py	100% <100%> (ø)
gapic/generator/generator.py	100% <100%> (ø)	⬆️

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