Add a mechanism that allows to install bash completion scripts to a standard location

It would be nice if packages could place bash completion scripts into some standard location, e.g. `~/.cabal/bash_completion.d/`, so that I can source all of them like so:

``` bash
if [ -d ~/.cabal/bash_completion.d/ ]; then
for file in `ls -d ~/.cabal/bash_completion.d/*`; do
. $file
done
fi
```