Function to display the current buffer in a WWW browser.
This is used by the `browse-url-at-point', `browse-url-at-mouse', and
`browse-url-of-file' commands.
If the value is not a function it should be a list of pairs
(REGEXP . FUNCTION). In this case the function called will be the one
associated with the first REGEXP which matches the current URL. The
function is passed the URL and any other args of `browse-url'. The last
regexp should probably be "." to specify a default browser.