Set description from CLI (for issue #73)

Just uses a -d/--description flag.

$ ./gist -h
Usage: gist [options] [filename or stdin] [filename] ...
Filename '-' forces gist to read from stdin.
    -p, --[no-]private               Make the gist private
    -d, --description [DESCRIPTION]  Set the description of gist
    -t, --type [EXTENSION]           Set syntax highlighting of the Gist by file extension
    -o, --[no-]open                  Open gist in browser
    -m, --man                        Print manual
    -v, --version                    Print version
    -h, --help                       Display this screen
    -c, --[no-]copy                  Copy gist URL to clipboard automatically