Refactor completions to allow adding custom functions (like git branches)

[DannyBen]

Instead of defining custom completions like this:

completions:
- file
- user

It is now required to use the explicit syntax, as used by completely:

completions:
- <file>
- <user>

This is done, so that we can easily use custom functions, for example, to show git branches:

completions:
- <file>
- <user>
- $(git branch 2> /dev/null)

In addition - the majority of the bash completion documentation in the README was removed, so that we only have one canonical documentation in the completely gem.