* md4c.h now contains macros MD_VERSION_xxxx.
* md2html has option --version.
(Of course those shall have some meaningful values only when we introduce
some release process.)
* Change API (md4c.h) to propagate different substring type info to renderer.
* Implement/refactor related code in the parser.
* Adapt renderer (md2html) to the new API.
Remove MD_SPAN_IMG_DETAIL::alt. Instead, the contents of the image is
propagated to the renderer via MD_RENDERER::text() callback.
* This fixes handling of entities inside the image text (issue #4).
* It simplifies parsing and, more importantly, it better distingusshes
what is responsibility of parser or renderer respectively.
* This allows more flexibility on renderers side. Renderer who do not
* really support images can just output the image content as any
other text.
The cost is a renderer into HTML (if it wants to render image contents
into the attribute ALT of the IMG tag), has to handle images with more
care. Typically such renderer has to track whether it is inside an image,
and if so, then render span enter/leave as an empty string.
With MD_FLAG_PERMISSIVEURLAUTOLINKS, we treat not overly complicated URLs
as autolinks even without '<' and '>'.
With MD_FLAG_PERMISSIVEEMAILAUTOLINKS, we treat not overly complicated
e-mail addresses as autolinks even without '<', '>' and without the
'mailto:' scheme.
Also expanded md2html utility and tests to cover these.
