Add usestrict option to include `'use strict';` in output

## Problem

I concatenate the output from ngtemplates into my application file. Part of my building process is to run JSHint after the concatenation, just in case. This step was failing because the generated function did not have `'use strict';` in there. I thought this might be helpful to other people, too!

## Solution

I added an option called `usestrict` (convention is no-underscores / all lowercase, right?) and if it is `true` then the generated function starts with a `'use strict';`. I also added a test case and updated the README.