Add configuration for setting up Run Script phases

This adds the ability for users to define custom Run Script build phases
to be added to the main application target. By default, we're only
adding the original TODO/FIXME comment script

- Adds `run_script_phases` key to configuration. The value for this key
  is an array of dictionaries. The key for the dictionary will be used
  as the filename of the template script and the value will be used as the
  name for the script inside Xcode.
- Removes the `install_todo_script` key from the configuration. This key
  is superseded by the `run_script_phases` key.

Fixes #118