support 6th interval, e.g. Am6.
Support 'sus' with no explicit interval.  E.g. Gsus9 (which is G9 sus4).

There's no need for us to capture this as anything other than text, for
now.
tidy and todo+
placate -Wall. minor.
tidy tests, and provide way to run the known-unsupported test cases
Support basic --html output.  Chords tagged with a classed span.
escape html in input if we are generating tagged output