Clarify pathname docs, follow spec with fragments (#2448)

* Clarify pathname docs, follow spec with fragments

- Valid URLs must not contain a `#` within its fragment.
  https://github.com/MithrilJS/mithril.js/issues/2445
- Our docs were a little confusing and misleading - `m.pathname` isn't
  aware of URLs, just path names.
- Removed the relevant extension to `m.parseQueryString` required to
  support the hash parsing extension. Now we just shave it off and
  ignore it.
- Fix support for arbitrary prefixes, so prefixes like `?#` are
  handled correctly.
- Add a bunch of tests to cover various areas of confusion and unusual
  edge cases.

* Update with PR [skip ci]

router/tests/test-getPath.js

@@ -6,7 +6,7 @@ var Router = require("../../router/router")
 
 o.spec("Router.getPath", function() {
 	void [{protocol: "http:", hostname: "localhost"}, {protocol: "file:", hostname: "/"}].forEach(function(env) {
-		void ["#", "?", "", "#!", "?!", "/foo"].forEach(function(prefix) {
+		void ["#", "?", "", "#!", "?!", "/foo", "?#", "##"].forEach(function(prefix) {
 			o.spec("using prefix `" + prefix + "` starting on " + env.protocol + "//" + env.hostname, function() {
 				var $window, router, onRouteChange, onFail