Add --force option to homesick rc command

Support automatically eval-ing .homesickrc file without prompting for user input.
This is particularly useful for headless scripts that do not support
user input.
Merge pull request #128 from wireframe/force-rc

Alright, in the absence of any further discussion I'm going to go ahead and merge this. There's one other feature I'm working on before cutting a new release and that will hopefully be done soon.