Don't autolink text if it's part of a previous inline

Previously it was possible to get the renderer to truncate the
output at a point that would cause it to output unbalanced tags,
for ex.:

`__o__@bar.com` -> `<strong>o</st<a href="mailto:__o__@bar.com[…]`

Not great. This commit attempts to mitigate that by tracking the end
of the last inline and making sure rewinds don't run past it. This is
basically the approach taken by @vmg in vmg/rinku@dc685c1

The only issue with this approach I can think of is that we can't autolink
bare emails like `__foo__@example.com`, but that'd be difficult to handle
without a proper way to rewind over inlines in the output buffer.