New directory for all bash-completion stuff; moved vagrant's completion script over.
Enable default file completion if no completion has been found.
Now respects bash_completion.d of homebrow.

Deleted local git scripts. The shell is now using those directly from
the respective folder where homebrew sets up the completion stuff.
New bash completion script for packer.

The completion script supports all commands and options of
packer-0.6.0 and is capable of completing options like
-<option>=(<value1>|<value2>).