Add sub-command 'purge'

I often see people on IRC and elsewhere suggesting that people `rm -rf ~/.cabal` which is far from optimal and actually in-sufficient.

I can however see the need to manually do _something_ before doing a complete `cabal install `, particularly for yesod based projects.

I am therefore proposing a `cabal purge` command that would DoTheRightThing(tm) so that people can say "just run `cabal purge`" instead of something more complicated and potentially wrong.

The `cabal purge` command would do the following:
- If there is a cabal.sandbox.config file, read it to get the cabal-sandbox location and the remove the directory:

```
${cabal-sandbox-location}/lib/${arch}-${os}-{$ghc_version}
```
- If there is no cabal sandbox config file, remove the directories:

```
~/.cabal/lib/${arch}-${os}-{$ghc_version}
~/.ghc/${arch}-${os}-{$ghc_version}
```

In each of the above two cases cabal would print:

``````
```
About to delete directories ...... [y/n]
```
``````

and when done print the age (see #2150) of the cabal index file.

Comments? Suggestions?

Start by locating cabal-install's command dispatch and sandbox configuration handling, then trace how cabal.sandbox.config identifies the sandbox location. Define the purge behavior for sandboxed and non-sandboxed projects, including confirmation prompts, architecture/GHC-specific paths, and reporting the cabal index age; the issue references #2150 for that age information.

Written by the indexing model from the issue text.