Support escaping of table characters inside a table

Nobody has claimed this yet.

A normal pipe table produces the desired table in MacDown.

```
test | a | table
--- | --- | ---
one | two | three
```

A pipe table with an escaped pipe symbol in the header is not recognized as a table.

```
test | P(a\|b) | table
--- | --- | ---
one | two | three
```

Please take a look at the [Babelmark output](http://johnmacfarlane.net/babelmark2/?normalize=1&text=test+%7C+a+%7C+table%0A---+%7C+---+%7C+---+%0Aone+%7C+two+%7C+three%0A%0Atest+%7C+p%28a%5C%7Cb%29+%7C+table%0A---+%7C+---+%7C+---+%0Aone+%7C+two+%7C+three) for this example. Desired output is currently produced by:

```
pandoc 1.17.1
PHP Markdown Extra 1.2.8
Minima 0.8.0a3_20140907
kramdown 1.2.0
cebe/markdown GFM 1.1.0
cebe/markdown MarkdownExtra 1.1.0
Blackfriday
Earmark 0.2.1
```

Reproduce the two pipe-table examples in MacDown and compare the rendering of the escaped-pipe case with the normal table. Trace the Markdown table parsing entry point used by MacDown; done means an escaped pipe in a table cell no longer prevents the table from being recognized, while ordinary table behavior remains unchanged.

Written by the indexing model from the issue text.