Add tg-static

With the `tg-static` attribute decorating a node, we can make sure that this node is not replaced during a fullpage refresh.

Though, if you were to refresh the page at a higher level -- e.g., refreshing an ancestor of the tg-static, the static aspect is no longer obeyed and it is replaced.

cc @tmlayton @alexaitken: the feature we need for a static navigation
cc @tylermercier @celsodantas @nsimmons @patrickdonovan @kurtfunai for thoughts and input on adding a new feature to TurboGraft