Bash completion

Phpbrew bash completion  featuring:

- phpbrew [tab tab]
- phpbrew use|switch|clean|remove|purge|download [tab tab]
- phpbrew ext [tab tab]
- phpbrew ext install|disable|enable [tab tab]
- phpbrew help [tab tab]

- `phpbrew install [tab tab]` completion for php versions and variants

PS: tested on Fedora for more than a month and totally forgot to commit and pull request. I guess this will probably work for other environments too.