Add support for passing test metadata

[pmeenan]

Add support for a "metadata" parameter that gets stored with the test data and echoed back in the JSON and HAR results as part of the page data.

It should allow for either JSON or string data and if JSON, decode it and echo back the attributes rather than a double-encoded JSON string.

It should have a size limit to make sure it isn't abused for storing arbitrary data (10k should be WAY more than enough).

For the HTTP Archive, this will be used for including the page ID and page rank with the test data, both of which are currently stored separately and joined at analysis time.