Check for .git directory before attempting copy

[finnhodgkin]

At the mo when our module is loaded it copies priv/pre-commit to the parent module's .git/hooks directory. Sometimes the parent module might not be an initialised .git repo, or it could be a subfolder of an initialised repo. Two solutions to this:

1. Run a check to see if the .git folder exists before copying - if it doesn't IO.puts an error message.
2. Add the above, but also add an optional second config variable that points to the absolute path of the correct .git/hooks.

[finnhodgkin]

Closed by #28