Expand bash completion capabilities

The previous bash completion file was somewhat limited and did not
include the ability to complete pacman specific options
(e.g, aura -S foo<tab><tab>). To enhance bash completion, the
following chages have been made:

Aura Only

* Expanded completion for all options and search sub-options
* Package completion for -M/--abssync
* Completion for orphans using self-generated list

Pacman

* Include completion for all pacman options
* Directory or file completion for some common options