Add zsh completion.

Move all completion files to completion dir.
Add branches to completion.

Add check for feature prune arguments.
Make vars local to prevent them from sticking around.

Sometimes hotfix branches would pop up in completion when using feature.
By switching the vars to local (rather than global), the user will never
see the hotfix branches again!
Zsh completion: fix wrong variable.

`_feature` was just a little *too* close to `_hotfix` - one variable
name didn't get changed over.
Use double quotes in general.

This is simply for good practice.
Revert "Use double quotes in general."

This reverts commit 9ba22bd6656693325a0cf86276d6cb893c18a445.

Double quotes are a no go with this version of zsh.
Readability: Add whitespace.
For the sake of security.
Merge add-zsh-completion (#86) into master

Add zsh completion.

Fixes #7.

Move all completion files to completion dir.