Add text-based browser dependency options. Text-based browsers are used
to render HTML. The dependencies are build and run dependencies, build
dependencies because nmh auto-configures itself at build time for text
based browsers it finds. The nmh upstream developer,
Ken Hornstein <firstname.lastname@example.org>, prefers w3m but any text-based
browser will do. The browser option is not default in order to satisfy
minimalist install rerequirements.
Reported by: Ken Hornstein <email@example.com> (upstream)