Allow configuring `cargo about generate` in `about.toml`

[pavpen]

As a cargo-about user, I want to be able to run cargo about generate in a crate's project directory without
having to figure out, and specify any other arguments, such as the path to the Web page template, or the
output path.

Notes

• One way to achieve this is to add configuration values in about.toml that correspond to providing
  the cargo about template, and output path on the command line.
• Another option would be to try a default value for the template path (such as about.hbs in the
  crate's project directory, and change the default output file from standard output to, e.g.
  license.html.

The first option is more flexible, doesn't require breaking backward compatibility, and seems
understandable for a user.