Features/add mappings generator command

Hi, I've created another command which allows you to create modman and composer mappings.

If this is something you would accept I'll write up some docs for it - otherwise I guess I could just fork the project or make it standalone.

Hopefully the descriptions should explain what it is doing.

If you would like to accept I'm open for any changes you might like to make - so go ahead and suggest any. 

Also note, I changed the package type back to "composer-installer" as "composer-plugin" seemed to cause issues when I did a composer update.