Add a $VROOMDIR environment variable containing the path to $VROOMFILE.

Using examples/basics.vroom as an example, $VROOMDIR might return
'examples', 'path/to/examples', or '.' (not the empty string), depending
on where vroom was run from.

Fixes #17.
Merge pull request #23 from malcolmr/vroomdir

Add a $VROOMDIR environment variable containing the path to $VROOMFILE.