Slate RT->MD converts Pills into MD Links

[t3chguy]

Reported by @tulir:

Rich text -> markdown converts pills into markdown URLs. Technically not a problem, but probably not what users expect.

[lampholder]

This one feels pretty weird :\ How easy is it to 'fix' (whereby I guess I mean 'special case pills to survived MD transformation back and forth)

[t3chguy]

We'd need our of slate-md-serializer instead of relying on https://github.com/tommoor/slate-md-serializer aiui

[t3chguy]

We no longer have the RT/MD dance.