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]

querystring/parse.js

@@ -2,12 +2,11 @@
 
 // The extra `data` parameter is for if you want to append to an existing
 // parameters object.
-module.exports = function(string, data) {
-	if (data == null) data = {}
+module.exports = function(string) {
 	if (string === "" || string == null) return {}
 	if (string.charAt(0) === "?") string = string.slice(1)
 
-	var entries = string.split("&"), counters = {}
+	var entries = string.split("&"), counters = {}, data = {}
 	for (var i = 0; i < entries.length; i++) {
 		var entry = entries[i].split("=")
 		var key = decodeURIComponent(entry[0])