Different emphasis delimiters

[shodgson]

Currently, emphasis is started and ended with either '*' or '_', but the same character should be needed.

Input:
*foo_bar*

Expected result:
<em>foo_bar</em>

Actual result:
<em>foo</em>bar*

From v0.28 of the CommonMark Spec:

9. Emphasis begins with a delimiter that can open emphasis and ends with a delimiter that can close emphasis, and that uses the same character (_ or *) as the opening delimiter.

[ttscoff]

That's fine and well for CommonMark, but with a rudimentary regex processor, I can't do this. If you were to use _**bold italic**_ my regular expression wouldn't pick it up properly if it was pairing the exact open and close. Lazy emphasis parsing isn't part of canonical Markdown, either.