Handle newlines in headings, table cells

[wooorm]

Subject of the issue

<h3>alpha<br>bravo and <td>alpha<br>bravo</td> can in no way be represented in Markdown (except by using HTML in Markdown).

The h1 and h2 versions can be used, if the heading is serialised as a Setext heading:

alpha\
bravo
===

Your environment

n/a

Steps to reproduce

<h3>NEW YORK<br>CHARLES E. MERRILL CO.<br>1907</h3>

Expected behaviour

A single space is probably fine. May need a handler.

Actual behaviour

### NEW YORK᛫᛫
CHARLES E. MERRILL CO.᛫᛫
1907