Frank export fails with .css-files in dynamic/css directory

[wallin]

Frank export fails if you have a .css file in the 'dynamic'-directory. I discovered this because I have scss-mode with auto-compile in emacs which (unecessarily) places a corresponding .css-file in the same directory:

./lib/frank/base.rb:185:in \`tilt': undefined method `new' for nil:NilClass (NoMethodError)

ext == '.css' doesn't exist in Tilt
185: Tilt[ext].new(filename)

[trvsdnn]

I've been meaning to handle this more gracefully.

Tilt is setup to determine which engine to render with based on the extension... I'll add a patch to just ignore files that don't match one of those extensions.