Support negated predef

[duncanbeevers]

In a .jshint config, you can negate globals via "predef".

This is useful in cases where you want to use jshint's dictionary of browser globals, but want to explicitly blacklist a subset of those entries.

{
  "browser": true,
  "predef": ["-event"]
}

[victorporof]

Is this an officially supported jshint thing? I had no idea. In any case, please also add some documentation in the readme about it.

[duncanbeevers]

It was an official feature, though predef is getting deprecated.

[duncanbeevers]

Pushed update to var usage.