Code review comment for lp:~jerith/divmod.org/short-directions-1215929

Commands currently do match prefixes, and the general `pyparsing.LineEnd()` in actions is a reasonable way to avoid doing this accidentally. We'll still either need an explicit `pyparsing.LineEnd()` in the `Go` expression or we'll need to move the `DIRECTION_LITERAL` option to the end of the expression. I'm inclined to go with the latter.