Add special handling for backslashed table pipes

[notriddle]

Fixes #356

This is an alternative version of #691 that follows GitHub's behavior more closely (it gives \| higher precedence in the parser than any other backslash escape).

Compare with the rendering in GitHub.

[Martin1887]

Thanks for your contribution!

I think #691 is more consistent with CommonMark and should be merged instead of this despite not following the GitHub behaviour, but it is a difficult decision.

What do you think, @raphlinus?

[raphlinus]

It is a difficult decision, as there are tradeoffs on both sides. I really wish there was a standardization effort that could address this.

Having wrestled with it, my personal feeling is to go with #699. It's arguably less clean, but it results in fewer distinct rendering possibilities for the same source markdown. But I'm also open to more discussion on the topic, and would encourage any effort to get this resolved more systematically, for example on the commonmark forums.

[Martin1887]

OK, I merge this because it solves and issue targeted for 0.10 and we can try to get more feedback on the CommonMark forums.

In a future version the decision could be changed, but it will probably be also different to #691.

Thanks!