Switch to using ZGEN_COMPLETIONS array

This is more consistent with the way plugin files are handled. It
will also make it easier to add other commands later to handle using
repositories that have completion files in them but no proper plugin
file. `zsh-users/zsh-completions`, for example.

It also keeps `${ZGEN_INIT}` a lot tidier since we can just add
`${ZGEN_COMPLETIONS}` to fpath in one op.
Make quoting consistent
Add completions command

Some repositories contain completion functions, but aren't formatted as
standard zsh plugins, like `zsh-users/zsh-completions`.

`zgen completions <optional subdirectory>` will clone a repo and add
it (or the subdirectory) to $fpath.
Merge pull request #5 from unixorn/add-dirs-to-fpath

Add `zgen completions` command
Add support for branches

Added an optional [branch] parameter to zgen-load and zgen-completions.
A branch name is appended to repository's path.
Update README.md