Matchers don't work with SSE response format

[davidvc]

Currently if you want to use matchers other than regular expression matching, a response body must be either JSON or XML.

A response format that is increasingly popular is Server Sent Events (SSE) as described here: https://html.spec.whatwg.org/multipage/server-sent-events.html

Server-Sent Events allow clients to be more performant and responsive as the full response does not need to be returned before the client can start processing any of the data.

The general format is one or more sections separated by two newlines, where each section looks like this:

event: <event-type>
data: <event-data>

Very frequently the data is JSON and could benefit from flexible, expressive matching. Instead today all we can use are regular expressions.

[uglyog]

This could be a good candidate for a plugin, see #83