Better handling of html templates/partials in make

Partial/template html files in src directory are no searched by
make automatically (no need to add them manually to the makefile).
They have to have the `*.html` endinng and must be in a path
containing the `partials` directory.

The copy process now works with multiple partials.
Merge pull request #87 from geoadmin/dev_make_templates

Better handling of html templates/partials in make