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
```

Start by tracing how Cabal installs package-provided files and how bash completion scripts are currently handled; the issue names no files or entry points. Define the installation location and package behavior, then verify that installed completion scripts can be sourced together from the proposed standard directory.

Written by the indexing model from the issue text.