Add config to pass arguments to grunt shell scripts

A few places we want to pass flags into shell scripts. This is easier than configuring entire new tasks.

If `grunt shell:task` runs `$ node ./foo.js`
Then `grunt shell:task:--flag:val` will run `$ node ./foo.js --flag val`
Merge pull request #441 from alphagov/feature/grunt-cli-args

Add config to pass arguments to grunt shell scripts