Add command for tabifying/untabifying all files in a project

[lee-dohm]

See: https://discuss.atom.io/t/tabify-all-files-of-project/19465

[harikt]

great! . Thank you.

[Julix91]

Would love this very much as well... like was noted in the linked conversation, replacing N spaces with tabs has many edge cases, which is why I'm using something to tabify in the first place. Now doing it across all files in a project would be amazing. :)