Add bash completion support

When the script can't complete arguments in a meaningful way (such as
for -v and -C), completion is disabled.

The bash completion rules use the ;& operator (introduced in bash 4.0)
for case-fallthrough.

Signed-off-by: Davide "FunkyAss" Del Zompo <davide.delzompo@gmail.com>
Merge pull request #208 from FunkyAss/bash-completion

Add bash completion support