Backticked pipes in table cells shouldn't delimit columns

[jbt]

I know there was already some stuff in #5 but I don't think it was fully resolved - it looks like 0076a1e doesn't actually fix the final case of a pipe char inside backticks that isn't escaped with a backslash

I've tried to do some reading around but I can't find any kind of consensus on what the Right Thing To Do here is:

 A |  B  | C 
---|-----|---
 1 | `|` | 3

As rendered by GitHub:

A	B	C
1	`	`

However markdown-it interprets the backticked | as a cell delimiter and breaks.

Looks like marked does the same thing, but there's an issue open there: markedjs/marked#285 - that hasn't been addressed though so I don't know what the "correct" behaviour should be.

(ref. jbt/markdown-editor#20)