Fixes dart-lang/markdowndart-lang/markdown#83.

An inline image is now rendered correctly, sans anchor element,
and not emitting a close tag for the image element. Unit tests
have been updated to match correct rendering.

An inline image may also be placed inside the content of a link,
and this case is also rendered correctly. I added three new unit
tests for this.

This *would* boost the CommonMark test for images from 1/22 to
9/22 and the test for links from 33/79 to 53/79, except the
CommonMark tests are sensitive to attribute order. (See separate
pull request on dart-lang/markdowndart-lang/markdown#51.)