Emphasis doesn't work within a word with _

[Aloso]

turndown uses _ by default for emphasis (italic text), but this doesn't work within a word.

Bru<em>ss</em>el<em>s</em> is spelled with 3 <em>s</em>'s.

• Rendered:

  Brussels is spelled with 3 s's.

• Converted to markdown:

  Bru_ss_el_s_ is spelled with 3 _s_'s.

• Markdown rendered with CommonMark:

  Bru_ss_el_s_ is spelled with 3 s's.

The problem is that CommonMark only recognizes underscores at word boundaries. To make emphasis work in the above situation, * should be used by default, which CommonMark recognizes even within a word:

Bru*ss*el*s* is spelled with 3 *s*'s.