Parse variable names with underscores

This is important for plugins, which declare variables with names like
`loaded_surround`. I have not bothered to make full VimL compliance;
e.g. while Vim also supports variable names like `plan9` and `_lodash`,
I have not bothered at this time to hammer that out (though that
shouldn't be hard; it just didn't strike me as important right now).
Merge pull request #1510 from Kazark/master

Parse variable names with underscores in them (e.g. for plugins)