Declare one-letter synonyms for some commands.
As issue #6 (Add single-letter verb synonyms) says:
Modern interactive fiction has converged on a fairly standard
vocabulary of one-letter abbreviations for common verbs. Besides
directional commands, they are:
d - drop
g - get
i - inventory
l - look
x - examine (synonymous in ADVENT with look)
z - wait
Open Adventure should implement these.
This commit doesn't make them work yet, but it declares them for when the
lexical analyzer is fully YAML-driven.