Add fetchGithubGraph (#204)

fetchGithubGraph is a tiny module which is responsible for fetching
GitHub contribution data, and parsing it into a graph.

Test plan:
The function is trivial and does not merit explicit testing.

src/plugins/github/fetchGithubGraph.js

@@ -0,0 +1,31 @@
+// @flow
+/*
+ * API to scrape data from a GitHub repo using the GitHub API. See the
+ * docstring of the default export for more details.
+ */
+
+import type {NodePayload, EdgePayload} from "./types";
+import type {Graph} from "../../core/graph";
+import fetchGithubRepo from "./fetchGithubRepo";
+import {parse} from "./parser";
+
+/**
+ * Scrape data from a GitHub repo, and return a SourceCred contribution graph.
+ *
+ * @param {String} repoOwner
+ *    the GitHub username of the owner of the repository to be scraped
+ * @param {String} repoName
+ *    the name of the repository to be scraped
+ * @param {String} token
+ *    authentication token to be used for the GitHub API; generate a
+ *    token at: https://github.com/settings/tokens
+ * @return {Promise<Graph<NodePayload, EdgePayload>}
+ *    a promise that resolves to a GitHub contribution graph
+ */
+export default function fetchGithubGraph(
+  repoOwner: string,
+  repoName: string,
+  token: string
+): Promise<Graph<NodePayload, EdgePayload>> {
+  return fetchGithubRepo(repoOwner, repoName, token).then((x) => parse(x));
+}