Added option to filter tweets by hashtag

Hi Ryan,

for a site I am currently working on, I needed a way to only display tweets that contain a certain hashtag, so I grabbed your great module and modified it to fit my needs.

Since I could not find a way to get the desired results from the twitter API (I tried searching tweets by author and hashtag, but unfortunately, this won't work for tweets older than 7 days), I basically grab all the tweets and send them through a function that does the filtering.

Since I prefered getting the plain data array and letting my template take care of the HTML, I also added a public getDataArray method.

I don't know if this is something you want to include in your module, but if you do, feel free to merge this PR :-)

Anyway, thanks for the great work, I am really happy using PW as my new CMS of choice..