Markdown rendering in image descriptions

[za3k]

Describe the bug
Markdown is being parsed and processed before being included as alt-text

To Reproduce

markdown2.markdown("![a*b*c](d)")

Renders as:

<p><img src="d" alt="a<em>b</em>c" /></p>\n

Expected behavior
I expect the alt text to be either abc (CommonMark spec) or a*b*c (no processing at all). Either seems okay.

Debug info
Version of library being used: 2.5.0 (or commit 8d3a65b)

Any extras being used: No