Add support for Font Awesome as a `fa:` icon set

[Chad Trytten]

​Font Awesome has become a widely used icon set across many different projects. A few notable examples include ​GitLab and ​Mermaid; incidentally, in both of these cases it's used in the diagraming that is also implemented as a macro hosted here. Not sure if this is a duplicate (sort of, but perhaps a specific solution rather than a generic) of 201, but since I used to create my own custom icons for many things before finding Font Awesome, perhaps this will meet many of the use cases? Font Awesome is the first listed icon set by ​w3schools.com - which also might provide some options for additional icon sets (re: 201) - and the site dedicates ​a whole page to the use of Font Awesome with many examples. The most intuitive shorthand usage across different tools seems to be fa:fa-car to represent the HTML <i class="fa fa-car"></i>; in contrast, ​MediaWiki seems to scope further, based on license, even though the namespaces do not collide per a ​search with all possible icons listed. Putting in this request only after implementing a lot of Font Awesome icons in various diagrams using the provided shorthand, which highlighted that the method of embedding so much HTML into a wiki page might be avoided by extending this plugin.

[Ryan J Ollos]

It sounds interesting. Would you consider creating a patch?