Support configurable persistent parameters.

New advanced config option `persistent-params`. When set, URLs are modified to include the parameters.

This implementation differs from #134 by supporting loads and prefetches in addition to navigations. Also, instead of the config being of the form `?abc=def&foo=bar` it omits the leading `?` because it may be appended to a URL that already has params. The leading delimeter is determined at runtime based on the presence of params on the given URL.

Closes #134
Does the config need to be prefixed with "`experimental-`" if it's an advanced option?