httpcaddyfile: Refactor to allow named matchers in route blocks

[francislavoie]

While updating caddyserver/website#71 I figured I'd take a look at what it would take to fix route to allow named matchers within it, turned out to be pretty simple.

There's was plenty of duplicate code in the logic of parseRoute compared to parseSegmentAsSubroute, but we didn't want the logic from buildSubroute for route because it does things we don't need.

It was pretty simple to extract out everything except the call to buildSubroute into an internal function so that route and handle can share almost entirely the same code paths.

(I recommend reviewing with "hide whitespace changes" turned on, makes the diff clearer)

[mholt]

Nice, this is an elegant change. Thanks for the test case. I don't see any problems with this so far so let's put it in the 2.2 beta and see how it flies!