Render html tags inside img alt as their original text

[rlidwka]

See discussion in #896.

Spec is not clear on how to handle this. Three variations exist:

$ echo '![text <textarea> text](image.png)' | /home/user/commonmark.js/bin/commonmark
<p><img src="image.png" alt="text <textarea> text" /></p>

$ echo '![text <textarea> text](image.png)' | /home/user/cmark/build/src/cmark
<p><img src="image.png" alt="text &lt;textarea&gt; text" /></p>

$ echo '![text <textarea> text](image.png)' | /home/user/.local/bin/commonmark
<p><img src="image.png" alt="text  text" /></p>

Discussion in commonmark spec issue is inconclusive.

In addition to that, there's a small inconsistency in markdown-it output (with html option on or off there will be different result), which is going to raise questions.

This is not necessarily a final solution (still waiting on spec), but the following change seems reasonable:

Prior to this commit:

• when HTML tags are enabled, tags were removed (as in Haskell version)
• when HTML tags are disabled, tags were escaped (as in C version)

After this commit:

• tags will be escaped (as in C version) regardless of HTML flag

Going with option 2 here. Not option 3, because people asked about this. Not option 1, because we escape <> inside attributes.

This commit also renders hardbreak token as newline - this is same as in commonmark reference implementation at this moment (not defined in spec).