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
If you want this to be an advanced config it should be prefixed with "advanced-". If not, be sure to add a default config into configs.js.

I could go either way with this one. It could cause problems if abused, but the problems are much clearer than some of the other advanced configs, so I don't think it would lead to many unexpected problems.