Helm source

Hi, thank you for your convenience program!

This PR's purpose is to use ghq from helm-for-files.
People can use helm-for-files with the ghq source by adding
helm-source-ghq to helm-for-files-preferred-list.
The configuration is like this:
```lisp
(add-to-list 'helm-for-files-preferred-list 'helm-source-ghq)
```
And then you can make sure by M-x helm-for-files after above configuration.
Please merge this pull request if you don't have problem.