Support for a custom preamble

[dblock]

I realized that #52 has this feature in it with specs, but you might want this feature in first.

You can generate a custom preamble using a code block. For example, you can replace the location of the original .coffee file by a relative one to Rails.root.

    Barista.add_preamble do |location|
        "/* : DO NOT MODIFY - compiled from #{Pathname.new(location).relative_path_from(Rails.root).to_s}\n\n"
    end

[Sutto]

Awesome, thanks!