Tweaks

I just realized I had made several handy tweaks to erc-tweet without
offering them back.  Sorry for the less than descriptive PR title.  :)

The changes are pretty self-contained commit-to-commit and I'd be
happy to split them into separate PRs if you'd rather have some but
not others.

The main changes are:
1. Mark only the new tweet text as read only.  I kept running into the
   case where I'd be typing a message and a tweet would come through
   and my unsent text would also be marked as read-only.
2. Handle mobile twitter URLs by removing the "mobile." from the
   beginning of the url before fetching the tweet.
3. Make the function that cleans up the tweet-text pluggable.  By
   default it stays as ```erc-strip-tags``` but this allows me to use w3m
   to render the tweets thereby rendering entities and such.  See the 
   commit message on e735d01 for that code- I didn't know if you'd want 
   it in erc-tweet directly, so I left it there.

Thanks!