Add context option for filter diffs

[kpengboy]

This adds a --context option to the diff subcommand and a --diff-context option to the apply and edit commands. The option allows the number of lines of diff context to be customized, which is helpful for verifying the correctness of changes to more complex filters.

[codecov]

Codecov Report

Attention: Patch coverage is 31.81818% with 15 lines in your changes missing coverage. Please review.

Project coverage is 54.52%. Comparing base (c566f95) to head (7dd5035).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/gmailctl/cmd/apply_cmd.go	0.00%	5 Missing ⚠️
cmd/gmailctl/cmd/diff_cmd.go	0.00%	5 Missing ⚠️
cmd/gmailctl/cmd/edit_cmd.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #421      +/-   ##
==========================================
- Coverage   54.63%   54.52%   -0.12%     
==========================================
  Files          53       53              
  Lines        4448     4457       +9     
==========================================
  Hits         2430     2430              
- Misses       1877     1886       +9     
  Partials      141      141              

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[mbrt]

Great stuff, left only minor comments, thanks!