Introduce a simple command that emulates `git hash-object`.

Introduce a simple command that emulates `git cat-file`.

Add a framework for commands to be defined, and add our first one,
"help".  When `git2_cli help` is run, the `cmd_help` function will be
invoked with the remaining command line arguments.  This allows users to
invoke `git2_cli help foo` to get information about the `foo` subcommand.