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

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).