1. lowercase tag names for better compatability with anal-retentive
    newer web standards.
 2. clean up the usage comment and example in the header.
 3. * In mkd2html, don't `#include <libgen.h>` unless the system has it.
    * Also in mkd2html, define a `basename()` if the system doesn't have it.
 4. Trim back leading spaces when returning fields from the document header.
 5. `#include <string.h>` in cstring.h so we can get the prototype for
    `memmove()`
 6. Update the mkd2html test program.
 7. Make the `<img` tag more xhtml-friendly by ending it with ` />`.
 8. `. . .` now maps to &hellip;, just like `...` does.
 9. Move the line trimming into `codeblock()` -- we need to be able to do
    trailing spaces on regular paragraphs.
10. Add **`MKD_QUOT`** ( expand `"` to `&quot;` ) to the list of
    user-settable flags.