Add an environment variable to change .now.json location

For issue #99

lib/cfg.js

@@ -2,7 +2,7 @@ import path from 'path';
 import fs from 'fs-promise';
 import { homedir } from 'os';
 
-const file = path.resolve(homedir(), '.now.json');
+const file = process.env.NOW_JSON ? path.resolve(process.env.NOW_JSON) : path.resolve(homedir(), '.now.json');
 
 export function read () {
   let existing = null;