Makefile: make POSIX-compliant
Use self-closing <br> tags

Use self-closing tags so that the HTML output is also valid XML.
Insert breaks for multiple consecutive blank lines

Insert <br> tags for consecutive blank lines, ignoring the first blank
Use aria-label for inaccessible <pre> text

Treat preformatted text with non-empty alt text as inaccessible and
provide the alt text in the aria-label attribute.
Don't output empty paragraphs
Don't output <br> tags
Rename to gmnitohtml
Update documentation
Update documentation
Allow specifiying a custom output template to use