Adding initial UI JS

This still needs lots of tests to be written, and checks for missing
browser functionality (specifically around Blob URLs)

src/index.html

@@ -1,10 +1,10 @@
 <div class="container">
-  <form id="regexp_form">
+  <form id="regexp-form">
     <textarea id="regexp-input" autofocus="autofocus" placeholder="Enter JavaScript-style regular expression to display"></textarea>
     <button type="submit">Display</button>
 
     <ul class="inline-list">
-      <li><a href="#" class="oi with-text" data-glyph="data-transfer-download">Download</a></li>
+      <li><a href="#" class="oi with-text" data-glyph="data-transfer-download" download="image.svg" type="image/svg+xml">Download</a></li>
       <li><a href="#" class="oi with-text" data-glyph="link-intact">Permalink</a></li>
     </ul>
   </form>