Add ability to specify multiple editors. A link for each editor is rendered.

@charliesome  My goal was to submit this with minimal code changes (easy to review) and preserve backwards compatibility so that users would not need to change their BetterErrors config. If you are interested in merging this then I'd like to continue develop the multiple-editor support. Thanks for the great gem.


![better-errors-multiple-editors](https://cloud.githubusercontent.com/assets/29266/4099182/686d332c-3050-11e4-81e1-4c4e86431c04.jpg)

* Support multiple editors.
* Use original link when only 1 editor is specified.
* Update existing specs.
* Rename methods to reflect plurality; add aliases for backwards compatibility.
* Update and add specs.