added history module

lib/history.js

@@ -0,0 +1,22 @@
+const shellEx = require('../lib/ex').shellEx;
+const handleError = require('./handle-error');
+
+const getDate = () => {
+    const d = new Date();
+
+    const months = ['January', 'February', 'March', 'April', 'May', 'June',
+        'July', 'August', 'September', 'October', 'November', 'December'];
+
+    return `${months[d.getMonth()]} ${d.getDate()}, ${d.getFullYear()}`;
+};
+
+module.exports = (currentVersion, newVersion) => {
+    const logCommand = shellEx(`git log ${currentVersion}..HEAD --pretty=format:"  * %s"`);
+    const date = getDate();
+
+    if (logCommand.code !== 0) {
+        handleError(logCommand.stderr);
+    }
+
+    return `###${newVersion} — *${date}*\n\n${logCommand.stdout}\n`;
+};