Support morpheme annotations

[dwhieb]

Sometimes scholars wish to make additional annotations on a gloss, beyond their actual glosses.

Example from Algonquian:

# Menominee
\txn  pemētaehkipew
\m    paemet-aehkw-ape
\gl   crosswise-face-sit
\gl-n initial-medial-final
\tln  S/he sits sideways.
\s    (Bloomfield 1975: 208)

\gl-n stands for "gloss note".

Just support one instance of this line for the time being, until a use case emerges for more.