Add support for providing alt text

To make published GIFs, via Micropub, more accessible, we should allow
alt text to be added to the images.

As per conversation on #3, this can't be reliably done through the
Gfycat API, so we should instead allow the user to provide this
information.

As we're using a JSON post, we need to construct an object for the
photo property as per https://micropub.spec.indieweb.org/#json-syntax

Closes #3.

server.js

@@ -178,7 +178,17 @@ app.get('/preview/:id', requireGfycatToken, (request, response) => {
 
 /* Login needed here */
 app.post('/post', requireLogin, (request, response) => {
-  var props = { photo: [ request.body.originalUrl ] }
+  var photo = []
+  if (request.body.altText && request.body.altText.length != 0) {
+    photo.push({
+      value: request.body.originalUrl,
+      alt: request.body.altText
+    })
+  } else {
+    photo.push(request.body.originalUrl)
+  }
+  var props = { photo: photo }
+
   if (request.body.inReplyTo) {
     props['in-reply-to'] = [ request.body.inReplyTo ]
   }

views/preview.hbs

@@ -19,6 +19,7 @@
         </label>
 
         <input type="hidden" name="originalUrl" value="{{ content_urls.largeGif.url }}" />
+        <input type="text" name="altText" placeholder="Alt text for the GIF, increase the accessibility of the post" />
         {{#if ../inReplyTo }}<input type="hidden" name="inReplyTo" value="{{ ../inReplyTo }}" />{{/if}}
         <input id="post-gif" type="submit" value="Post this GIF" />
       </form>