Configure template file extensions

[jambonrose]

During conversation of #4, several people expressed the desire to report missing templates that do not end in .html or .htm, as some templates end in .txt or arbitrary extensions like .subject.

We should probably default to .html, .htm, and .txt (this last is used by Django emails). It should be possible for developers to configure the extensions being matched by the find_executable_files() method, so that they can add arbitrary extensions like .subject if necessary.

[alexandervlpl]

Great work on this plugin! In my project, the Django template engine is also used to generate PDFs. These templates are LaTeX files with .tex extensions. Seeing them in coverage reports would make this even more awesome.

[nedbat]

This is implemented in 07970d9

[nedbat]

This is now released in v2.0.0: https://pypi.org/project/django-coverage-plugin/2.0.0/