Added support for predefined variables

Hi there Darren,

I installed your jslint plugin yesterday, which I found pretty useful, but wasn't able to find the way to pass some global variables to JSLint.

Inspecting the code, I saw that JSLint accepted predef variables in the form of either an Array or and object (line 6068), so I just made a pretty straigthforward change in linter.js to accept a "predef" argument followed by a string containing a javascript syntax compliant array of strings.

Do as you wish with this commit.

Regards.