Add more chord types

[noeddl]

More chords, more fun:

• [x] Augmented triads, e.g. Caug/C+
• [x] Diminished triads, e.g. Cdim/C0
• [x] Augmented seventh chords, e.g. Caug7/C+7
• [x] Diminished seventh chords, e.g. Cdim7/C07
• [x] Half-diminished seventh chords, e.g. Cm7b5
• [x] Major seventh chords, e.g. Cmaj7/CM7
• [x] Minor major seventh chords, e.g. CmMaj7/CmM7
• [x] Augmented major seventh chords, e.g. CaugM7/C+M7
• [x] Suspended-second chords, e.g. Csus2
• [x] Suspended-fourth chords, e.g. Csus4

To come up with this list, I used mainly these two sources:

• https://en.wikipedia.org/wiki/Chord_(music)#Common_types_of_chords
• https://ukulele-chords.com/

[noeddl]

I became a bit tired of adding chord types all the time, so I moved some of them into a new issue to be tackled later (#35).