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

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.