Filter snippets by tag -> pet list -t $tag #329

[a2xdeveloper]

Issue #, if available:
#329
Description of changes:

Added support of tag filtering on list command

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[RamiAwar]

Thanks for this! Added a few small touches:

• Tests
• Changed function to return the filtered snippets instead of changing in-place. This is a bit more like Go philosophically - more functional, less susceptible to bugs. Changing in-place is tricky.
• Used the new stdlib "slices" to call Contains instead of defining one ourselves.