With this change, autolinks are parsed as Links with
the `uri` class. (The same is true for bare links, if
the `autolink_bare_uris` extension is enabled.) Email
autolinks are parsed as Links with the `email` class.
This allows the distinction to be represented in the
URI.
Formerly the `uri` class was added to autolinks by
the HTML writer, but it had to guess what was an autolink
and could not distinguish `[http://example.com](http://example.com)`
from `<http://example.com>`. It also incorrectly recognized
`[pandoc](pandoc)` as an autolink. Now the HTML writer
simply passes through the `uri` attribute if it is present,
but does not add anything.
The Textile writer has been modified so that the `uri`
class is not explicitly added for autolinks, even if it
is present.
Closes#4913.
