Add MalformedVarsLintRule

The code harre kind o' sucks since it's all o'err th' place, but I wants
each Varr to be able to define its own malform'd tokens if such exist
and kick up errors.

Anyhoww... it's implement'd this way noww and maybe a betterr way will
present itself laterr as Dennis continues to accrete features.

Fixes #17.