This change adds a way to actually configure the RazorEngine to use
extensible directives (previously buried behind legacy API). As part of
this feature adds the RazorParserOptions class to encapsulate anything
else that becomes a parser options (ahem taghelpers).
Now we have a pattern for this when we get there.
Options are propagated as part of the RazorSyntaxTree for
testability/sanity and this was actually responsible for the bulk of the
changes.
Also added some extension methods for adding directives to the
IRazorEngineBuilder and an end to end integration test.
Ryan Nowak