Return URL params in HandlerOptions

[nehzata]

@alecthomas What's your position on this change? I've come across a case where an Options Handler needs access to information contained in the url params.

eg:
//happy:api GET /v1/metrics/:orgId authenticated permissioned

In this case the permissioned option handler needs access to :orgId in the URL to determine if the authenticated user has access to that org.

This PR changes the []string of params to a map[string]string and merges it's content with the options map. I feel it's a little hacky but to fix it properly required substantial more work. Next stage I think we should factor out the route, params & body parsing section out as it's kind of duplicated in ServeHTTP and HandlerOptions..

[alecthomas]

Seems reasonable to me!