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.