Support configurable persistent parameters.

New advanced config option `advanced-persistent-parameters`. When set, URLs are modified to include the parameters.

Parameters are appended prior to loads and prefetches in addition to navigations. The format of the config should be "abc=def&foo=bar" without a leading "?". The leading delimeter is determined at runtime based on the presence of params on the given URL.

Closes #134
FWIW, the comment here comes after `isAllowed_` in this function, but the call to `appendPeristentParameter` comes before `isAllowed_` in `handleClick_`