Script the release process to make it easier to build a tar containing
the executable, Bash completion, and the man page.

Instead of the messy variables we were using, which caused
`passextract.1.txt.tmp` and `passextract.1` files to be generated at the
root of the project, create a few new targets to do the "`"` to "*"
transformation and build the man page from a temporary file.

To more clearly identify literals in "`literal`" blocks in the man page,
make them bold by doing a simple text replacement on them.

Note that this Makefile makes a `passextract.1` file at the root of the
project. Need to fix that.

Provide a man page for in situ documentation. Use AsciiDoc to generate
the man page.

Yay, my first man page!

Resources:
- https://github.com/BurntSushi/ripgrep/blob/fe9be65/build.rs
- https://github.com/BurntSushi/ripgrep/blob/f5411b9/doc/rg.1.txt.tpl
- http://www.methods.co.nz/asciidoc/asciidoc.1.txt
- https://github.com/git/git/blob/23f8239/Documentation/git-clean.txt
- https://github.com/git/git/blob/8a8f121049fdd881981799755a041b3f81449805/Documentation/asciidoctor-extensions.rb
- http://asciidoctor.org/docs/asciidoc-syntax-quick-reference/
- https://github.com/git/git/commit/5945717009cbd07fc303939452df6475ac342fe7

Previously, when the "invisible" flag was activated, we wouldn't print
anything except the "p: " prefix on a password line. This made it
difficult to verify the password, and if you for some reason had
multiple passwords, you wouldn't know which one to pick unless you knew
the order.

This at least gives you a visual cue about your password.