Added a coffeescript guard style error output.

I also use coffescritpt guard, one of the features the have is outputting errors directly to the supposed file. So I added `:error_to_haml` option that allows user to specify the following in the Guard file:

    :error_to_haml => true

in this case and all Haml errors will be routed to the output file. Otherwise it will behave the same way.