Website: Add code licensing info to footer.
Merge pull request #174 from nicksay/web-footer

Website: Add code licensing info to footer.
Website: Use variables for release versions on Downloads page.

Using variables simplifies linking to the latest release to changing
the values in the `_config.yml` file.

Closes #182.
Merge pull request #184 from nicksay/web-downloads

Website: Use variables for release versions on Downloads page.
Update SPF short description.
Merge pull request #186 from nicksay/update-tagline

Update SPF short description.
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