Markdown reader: Fixed link parser to avoid exponential slowdowns.

Previously the parser would hang on input like this:

    [[[[[[[[[[[[[[[[[[hi

We fixed this by making the link parser parser characters
between balanced brackets (skipping brackets in inline code spans),
then parsing the result as an inline list.

One change is that

    [hi *there]* bud](/url)

is now no longer parsed as a link.  But in this respect pandoc behaved
differently from most other implementations anyway, so that seems okay.
All current tests pass.  Added test for this case.

Closes #620.

0 files changed, 0 insertions, 0 deletions