There are various cases in which inline text fails to be separated by
(sufficiently many) newlines from adjacent block content.  A paragraph
needs a blank line (two newlines) separating it from prior text, as
does an underlined header; an ATX header needs a single newline
separating it from prior text.  A list needs at least one newline
separating it from prior text, but in general two newlines (for an
ordered list starting other than at 1, which will only be recognized
given a blank line before).

To avoid accumulation of more newlines than necessary, take care when
concatenating the results of converting consecutive tags to remove
redundant newlines (keeping the greater of the number ending the prior
text and the number starting the subsequent text).

This is thus an alternative to matthewwithanm#108 that tries to avoid the excess
newline accumulation that was a concern there, as well as fixing more
cases than just paragraphs, and updating tests.

Fixes matthewwithanm#92

Fixes matthewwithanm#98