Use memchr() instead of strchr() when checking if a char's in a group

strchr() may return non-NULL if the second arg is '\0', since
it considers the terminating '\0' in the first arg to be part of the
string.
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 rinku:

vmg/rinku@dc685c1
Add changelog entry