refactor: rename app id to client id (#12)

Renaming the parameter appId to clientId. Using client id will make it
more obvious to users what the value corresponds to in the Unsplash API,
as this should really be a layer on top of that API.

BREAKING CHANGE: appId is no longer supported and users will need to use
clientId for the same value.

README.md

@@ -14,18 +14,16 @@ Before using the Unsplash API, you need to [register as a developer](https://uns
 
 ```javascript
 // In your gatsby-config.js
-// Application id provided by Unsplash
+// Client id refers to the access key provided by Unsplash
 // Collections must be public to return photos
 plugins: [
   {
     resolve: `gatsby-source-unsplash`,
     options: {
-      appId: `12345678`,
-      collections: [
-        `098765`
-      ],
+      clientId: `12345678`,
+      collections: [`098765`],
       // optional: will only get page 1, so increase this count to include > 10 photos
-      perPage: `100`
+      perPage: `100`,
     },
   },
 ];
@@ -37,45 +35,45 @@ Get all photo urls in reverse chronological order with the photographer's name:
 
 ```graphql
 query PhotosQuery {
-    allUnsplashPhoto(sort: { fields: [created_at], order: DESC }) {
-      edges {
-        node {
+  allUnsplashPhoto(sort: { fields: [created_at], order: DESC }) {
+    edges {
+      node {
+        id
+        user {
           id
-          user {
-            id
-            name
-          }
-          urls {
-            full
-            regular
-            small
-          }
-          description
-          created_at
+          name
+        }
+        urls {
+          full
+          regular
+          small
         }
+        description
+        created_at
       }
     }
+  }
 }
 ```
 
 Get a specific photo's urls by id with links to photographer's Unsplash profile:
 
 ```graphql
 query PhotoQuery {
-    unsplashPhoto(id: { eq: "GnY_mW1Q6Xc" }) {
-        id
-        description
-        urls {
-            full
-            regular
-            small
-        }
-        user {
-            name
-            links {
-                html
-            }
-        }
+  unsplashPhoto(id: { eq: "GnY_mW1Q6Xc" }) {
+    id
+    description
+    urls {
+      full
+      regular
+      small
+    }
+    user {
+      name
+      links {
+        html
+      }
     }
+  }
 }
 ```

gatsby-node.js

@@ -2,14 +2,17 @@ const Promise = require('bluebird');
 const axios = require('axios');
 const crypto = require('crypto');
 
-exports.sourceNodes = ({ actions }, { appId, collections, perPage = 10 }) => {
+exports.sourceNodes = (
+  { actions },
+  { clientId, collections, perPage = 10 }
+) => {
   const { createNode } = actions;
   return Promise.all(
     collections.map(collection => {
       return axios
         .get(`https://api.unsplash.com/collections/${collection}/photos`, {
           params: {
-            client_id: appId,
+            client_id: clientId,
             per_page: perPage,
           },
         })