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
line.

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

Add README.md

Add documentation

Initial commit