seaborn.objects: Add formatting string argument to Text mark

[qiemem]

Adds a fmt argument to the so.Text mark, allowing the user to pass in a format string to format the text before display. I also considered having it just take in a function, similar to .label's arguments. Let me know if that would be preferred.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.32%. Comparing base (3ef2a0f) to head (f560cbe).
⚠️ Report is 88 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3427   +/-   ##
=======================================
  Coverage   98.32%   98.32%           
=======================================
  Files          77       77           
  Lines       24303    24309    +6     
=======================================
+ Hits        23897    23903    +6     
  Misses        406      406           

Files with missing lines	Coverage Δ
seaborn/_marks/text.py	100.00% <100.00%> (ø)
tests/_marks/test_text.py	100.00% <100.00%> (ø)

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[JanPalasek]

@mwaskom is there some problems with this PR?
For instance I find myself needing this functionality in a lot of plots. To bypass this, I use apply with pandas to create a new formatted column.