Adding an additional multi task

I'm working on a node application where the client and server side javascript need slightly different JSLint directives, for example client side has browser:true, where server side has node:true and sloppy:true.

This PR adds multiTask support to grunt-jslint so that I can easily setup two separate tasks with different directives to lint each section of JavaScript.

There doesn't seem to be a way to upgrade a standard single task to support multi task, so for the sake of backwards compatibility, I've left the old task alone and added a new task called "jslintm".

This way all old jslint tasks carry on functioning, but anyone who needs to do split linting like me can add or upgrade their tasks.