Generate list of supported YANG models for documentation

[troglobit]

As discussed in #326, after the big refactor to offline, we could generate a list of supported YANG models, their features, as well as augments/deviations.

As an example, with only a few models, here's how to create a HTML/JavaScript page of IETF System:

pyang -f jstree [email protected] [email protected] \
                [email protected] > infix-ietf-system.html