Markdown: backslashes enclosed in backqoutes inside/outside table

[nobu]

For this markdown, backslashes are rendered differently, with 6.14.2 and master(e4d727f).

# Backslashes

## Plain

`` `$\ ` ``   : `$\ `

`` `$\\` ``   : `$\\`

`` `$\\\\` `` : `$\\\\`

## Table

|  plain        | code    |
| :-----------: | :-----: |
| `` `$\ ` ``   | `$\ `   |
| `` `$\\` ``   | `$\\`   |
| `` `$\\\\` `` | `$\\\\` |

Related to #1386.