Extract IGNORE_MARKER - used in templates.

Refactor handling of lines to ignore in templates. Currently only used
by the Pull class, but could clean up things elsewhere.
Refactor diff filetype detection in Pull::Diff

Just use a file ending...
Add .ghi endings to Pull template files

Can be detected by editors - the soon to be released vim-ghi plugin is
an example for that.
Use the IGNORE_MARKER inside the templates as well
Make use of IGNORE_MARKER in Editor

... to strip away everything we want ignored.