fix autocompletion for short commands

[ingkebil]

Fixes #38

Checklist

• I have made a change to this repository, be it functionality, testing, documentation, spelling, or grammar.
• I updated my branch with the master branch.
• I have added the necessary testing to prove my fix is effective/my feature works (or I did not modify functionality).
• I have added necessary documentation about the functionality in an appropriate .md file.
• I have appropriately commented any code I have modified

Short description of what this PR does:

• The short commands do not autocomplete file names. e.g. notes o <tab><tab> does not yield the files in your $NOTES_DIRECTORY. This PR adds that autocompletion.
• The search and ls commands were omitted from the autocomplete. This has been added and is covered by the unit tests.

I wasn't sure if this needs extra unit tests. Let me know what you think!

[pimterry]

Nice work! This looks great to me, merging now. Thanks for contributing 👍