fix html-safety bug

the existing test happened to pass because the safe string happened to
be at index 1 :facepalm:
add ability to flag interpolation values as html-safe

in addition to being able to use an html-safe placeholder (e.g.
%h{safe}), you can also pass in an I18n.HtmlSafeString as an
interpolation value and it will be preserved
expose CallHelpers at runtime

this way you can call inferArguments, override stuff, etc.
v0.0.7