whitespace
Only mark the new tweet text as read-only

I believe everything else should already be read-only and going to
point-max had the side effect of marking any unsent text at the erc
prompt as read-only as well, which was a hassle.
handle mobile urls

This works by altering the tweet url regex to match mobile urls and then
"correcting" those urls by removing "mobile." from them.
use point-max-marker
move extracting the tweet-text into its own fn
make the tweet 'renderer' configurable

This lets someone replace the default text renderer with something a
little more powerful, for instance:

(defun erc-tweet-w3m-cleanup-text (tweet-text)
  (if (featurep 'w3m)
      (with-temp-buffer
        (insert tweet-text)
        (w3m-region (point-min) (point-max) nil 'utf8)
        (concat (replace-regexp-in-string
                 "\n" ""
                 (buffer-substring-no-properties
                  (point-min) (point-max)))
                "\n"))
    (erc-strip-tags tweet-text)))
(setq erc-tweet-cleanup-text 'erc-tweet-w3m-cleanup-text)
constrain the regex for twitter urls a bit more.

It doesn't match http://foomobile/twitter.com...
Added tweet render example.