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!