Allow passing local JSON instead of downloading

This way, if you have the JSON already there locally, you don't
have to bother downloading it from online. Especially useful if
the JSON is passed to you some other way, like through a browser
for example

functions.js

@@ -31,13 +31,19 @@ class Seance {
     */
   }
 
-    async fetchFromMedium (mediumUrl) {
+    async fetchFromMedium (mediumUrl, options = {
+        json: null,
+      }) {
       console.info(`Fetching: ${mediumUrl}`);
       var output = path.join(process.env.PWD, 'content')
 
       var json
 
+      if (!options.json) {
         json = await this.fetchMediumJSON(mediumUrl)
+      } else {
+        json = options.json
+      }
 
       // use mediumexporter's getPost function to fetch a Medium post
       const post = await getPost(mediumUrl, {
@@ -264,8 +270,10 @@ class Seance {
       mediumUrl= `${mediumUrl}?format=json`
       const response = await fetch(mediumUrl)
       text = await response.text()
+    } else if (fs.existsSync(mediumUrl)) {
+      text = fs.readFileSync(mediumUrl).toString()
     } else {
-      throw { error: 'URL must be a Medium URL' }
+      throw { error: 'URL must be a Medium URL or existing JSON file' }
     }
 
     json = await JSON.parse(text.substr(text.indexOf('{')))