Probe command (fix issue #33)

Here's a first take at adding the probe command. Please review and comment.

How can we use the MockServer to add tests?
Initially I've read it this way: "fetch `<url>` and return response headers that have `<text>` in their values". It is not clear which headers are returned (request or response), and what is the content - it should be mentioned that it is the contents of a fetched webpage. 

It'd also be good to describe the use case and the problem in more details, something like "some pages vary their contents based on request headers; this command tries to find a working set of headers by making several requests (using different headers) and checking if a specified text is in the returned page contents".