Read template `popup.wflow` from the same directory as `index.js`.

The CLI wasn't finding `popup.wflow` unless I was in `/usr/local/lib/node_modules/website-popup/`.

> ```shell
~ $ website-popup 'https://google.com' --size 800x600
fs.js:443
  return binding.open(pathModule._makeLong(path), stringToFlags(flags), mode);
                 ^
Error: ENOENT, no such file or directory 'popup.wflow'
    at Error (native)
    at Object.fs.openSync (fs.js:443:18)
    at Object.fs.readFileSync (fs.js:309:15)
    at Object.<anonymous> (/usr/local/lib/node_modules/website-popup/index.js:4:14)
    at Module._compile (module.js:449:26)
    at Object.Module._extensions..js (module.js:467:10)
    at Module.load (module.js:349:32)
    at Function.Module._load (module.js:305:12)
    at Module.require (module.js:357:17)
    at require (module.js:373:17)
```