Fix editing remote unwriteable file

With the original advice, if we try to find a not writeable file on a
remote machine, the extra tramp prefix would be inserted. As a result,
we actually end up in a blank new file.
Merge pull request #627 from toctan/sudo-tramp

Fix editing remote unwriteable file
A better `prelude-view-url`

- remove HTTP headers, what we care most of the time is the content
- `set-auto-mode` is way better than the naive xml, html tag check
Update README
Merge pull request #628 from toctan/view-url

A better `prelude-view-url`
Merge remote-tracking branch 'upstream/master'