don't test coverage on plugins
since it's not easy to cover code that use external services like
Evernote cloud and calibre.
add option to export highlights to local HTML files in Evernote plugin
Merge pull request #1004 from chrox/html_highlight

add option to export highlights to local HTML files in Evernote plugin
use new FFI based mupdf API

This will use the new API where the Lua/C interface was used before
and also drop usage of mupdfimg wrapper which is now implemented
within the mupdf interface