Adding environment variable for editor protocol.

## Problem

Not everyone uses Sublime editor so the links to edit a failing test in Sublime aren't universally useful.
I would like to have a way to 

## Solution

Allow the user to specify which editor (via the protocol) they wish to use.

## Implementation

* I defaulted to Sublime in the event you don't specify anything.
* I "patch" in the `EditorProtocol` value to the context in the `render` function. This works in practice, but feels sort of dirty because we aren't being super explicit. The alternative would be to explicitly add `EditorProtocol` to the context before calling `render`. Let me know what you think.