Try to register all javascript files found in themes as handlebars
helpers.
Note that, unlike all other theme files currently, format directories
are ignored. I don't think there's a use case for format-specific
helpers, and this gives theme developers the flexibility to put them
either in top level files or organize them in subdirectories however
they see fit.
Note also that the theme format seems to be primarily documented in
<https://github.com/fresh-standard/fresh-themes>. This newly recognized
theme file type should be documented there should this branch be merged.
It seems that some time in the last couple years wkhtmltopdf's default
margins were changed from '10mm' to zero. As an alternative to #177,
this PR adds an option to pass in arbitrary wkhtmltopdf long arguments
and sets the default top and bottom margin to '10mm'.
ryneeverett