Support username:pass authority in HTTP/WS urls

PUBLISHED_FROM=39a1c2a271c5cd961670e11c830105c17ba0b2e4

docs/c-api/http_server.h/mg_parse_http_basic_auth.md

@@ -0,0 +1,13 @@
+---
+title: "mg_parse_http_basic_auth()"
+decl_name: "mg_parse_http_basic_auth"
+symbol_kind: "func"
+signature: |
+  int mg_parse_http_basic_auth(struct mg_str *hdr, char *user, size_t user_len,
+                               char *pass, size_t pass_len);
+---
+
+Parses the Authorization: Basic header
+Returns -1 iif the authorization type is not "Basic" or any other error such
+as incorrectly encoded base64 user password pair. 
+