Create option to set Gist filename and add Tweet Text label.

[MilesMcBain]

Adressing #2.

I added the Tweet Text label since I got a question on Twitter about how to customise the tweet text. I guess it can look like the text box is just for the Gist URL and people don't realise you type the tweet in there.

I placed the Gist name update in the tweet button handler, since intermediate filename updates are irrelevant and also triggering the update in another way was either unreliable (from text input change) or required another button.