Adding support to ignore some file paths during offline compression

When using compressor with current satchmo stable code, we faced some errors in google checkout payment confirm template because google checkout module is not enabled for the project. This is happening because compressor iterates over the templates directories of all installed apps even if they are not being used or overridden in project.

So we had to make this fix to fix the above satchmo error. As an additional benefit, we are now also using it to improve the speed of compress command by ignoring template directories which do not support compress or are not being used.