Added basic infrastructure for render_stack.
Use stack approach for rendering strong, emph.
Use regular strings for literal in render_stack.
Use render stack for inline links.
Spec: use plain string content of label for alt text in images.

So, `foo bar` rather than `foo *bar*` or `foo <em>bar</em>` or
`foo &lt;em&gt;bar&lt;/em&gt;`.
HTML renderer:  render alt text as plain, unformatted string.
Updated js writer to render plain text for alt attributes of images.