fastn.font declaration change

[amitu]

We currently allow fonts to be declared this like:

-- fastn.font:  Montserrat
style: normal
weight: 400
woff: /assets/fonts/static/Montserrat-Regular.ttf

The "identifier" part, which is in the "caption" field. We should follow -- <type> <identifier>: syntax for consistency sake:

-- fastn.font Montserrat: 
style: normal
weight: 400
woff: /assets/fonts/static/Montserrat-Regular.ttf

And possibly switch to lower case nomenclature for font names.