Backward compability

I have been using shell-pop for a full year now, relying on the `el-get` recipe. I went to see the code to see how to add some feature when I discover this repo, and realize el-get  was pointing to the long outdated emacswiki version.

Using the new version I had to totally rewrite my shell-pop config. Updating el-get recipe would have so break a lot of user config. 
To prevent this, I reintroduce the config function in the legacy emacs wiki, keeping the same API, but changing their code to work with current version.  Doing so I refactored the custom, to have the `shell-type` définition in a single place.