Extract a twitter screen name from a twitter URL or @username
string. Return null
if not valid.
var twitterScreenName = require('twitter-screen-name')
twitterScreenName('http://twitter.com/_alanshaw') // -> _alanshaw
twitterScreenName('https://www.twitter.com/_alanshaw') // -> _alanshaw
twitterScreenName('http://twitter.com/_alanshaw/status/695527644901801984') // -> _alanshaw
twitterScreenName('@_alanshaw') // -> _alanshaw
twitterScreenName('_alanshaw') // -> _alanshaw
// etc.
twitterScreenName('http://example.org/foo/') // -> null
twitterScreenName('toolongfortwitter') // -> null
// etc.