Add `before` and `after` options to insertIntoFile

Adds the ability to specify the string in a file to insert contents before or after, similar to the [same method in Thor](https://github.com/erikhuda/thor/blob/master/lib/thor/actions/inject_into_file.rb#L17).

 Example:
 ```
 // .jshintrc
 "predef": [
   "window",
   "document"
 ]
 
 insertIntoFile('.jshintrc', '  "newGlobal",', {after:'"predef": [\n'});

 // new .jshintrc
  "predef": [
    "newGlobal",
    "window",
    "document"
 ]
```