feat(rxPageTitle): Method to optionally strip HTML from page title

Note that this PR will not change the default behaviour of anyone already calling `rxPageTitle.setTitle`. It adds a new method `rxPageTitle.setTitleUnsafeStripHtml`, to make it explicit that the source of the title must be trusted.

It does add a new `unsafe-html-title` attribute for `rxPage`, if you wish to pass arbitrary HTML to the page title and have it internally call `rxPageTitle.setTitleUnsafeStripHtml`

Closes #660