#115: bin/base64: modernize the demo program

The library has long contained a demo program, `bin/base64.c', that
gives a working demonstration of how the library could be used. The
program intentionally mimics the behavior of GNU Coreutils `base64'.

This program is pretty old and is not as polished or well-written as it
could be. Improve it in the following ways:

- Use `stdbool' types for returning success/fail from functions.
- Add more comments to explain what is going on.
- Add proper command line options parsing, and add a help function.
- Default to printing encoded output in 76-char columns, like GNU
  Coreutils `base64'.
- Print all error messages to `stderr'.

Resolves #115.