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.
Rebind Handlebars helpers to drop the pesky options hash for standalone
helpers that don't need it. Move block helpers (which do need the
Handlebars options/context) to a separate file for special handling.
