Allow setting config URL with <link> (#1146)

* fix: allows for valid config types expansion

* feat: config url can now come through link tag

* fix: lints added coded

* fix: slims down code per review

* fix: expands query to find supported type

* fix: removes typo in test copy

* fix: changes validTypes to object

* fix: groups config functions into one getConfig func

* adds console message for config url

* adds to docs

* update docs

* fix test

* fix merge conflicts

contributor addition moved to #1241

* avoids empty path with link without href. changes link console message

* removes additional console

* fixes link path in console

* fix: remove superfluous .allcontributorsrc change

example/index.html

@@ -7,6 +7,15 @@
 
   <link href='https://fonts.googleapis.com/css?family=Roboto:300,400,500' rel='stylesheet' type='text/css'>
   <link rel="stylesheet" href="/cms.css"/>
+  <!--
+  Netlify CMS will automatically look for a "config.yml" file in the same directory
+  as this "index.html", but you can override this by providing a link tag like this
+  one:
+  
+  <link href="path/to/config.yml" type="text/yaml" rel="cms-config-url">
+-->
+
+
   <script>
   window.repoFiles = {
     _posts: {