Use custom URL Scheme in web UI to allow compatibility with any editor

## 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 specify my editor (Mac Vim, etc).

## Solution

Allow the user to specify which editor (via the protocol) they wish to use.

For example:

`$ GOCONVEY_EDITOR_PROTOCOL="mvim" goconvey`
`$ EDITOR_PROTOCOL="subl" goconvey`

Or just export it in your `~/.bashrc` or something and be happy.

## 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.

This implementation doesn't try to be clever at all: ![image](http://i.imgur.com/oIl1M2H.png)