Add 'append' and 'overwrite' options

Scaffolded files now have several more options for writing files and dependencies:

* **create** - Create the file if doesn't exist. Skip if it does (default).
* **overwrite** - Always create the file from the scaffold, even if the destination filename exists.
* **append** - Append to an existing filename (i.e. for routes, controller actions, etc.)

Possibly a "prepend" or search option might make sense in the future too. I've got created a separate branch for brunch-with-chaplin that takes advantage of this new feature at paulmillr/brunch-with-chaplin#63