Add `--meta` switch to the `show` command

If `--meta` is passed, only the password's metadata will be
displayed---i.e., everything from the second line onwards.

Password file `Website/Example`:

    mysecretpassword
    URL: http://example.com
    Username: John Doe

Example runs:

    user@host ~ $ pass Website/Example
    mysecretpassword
    URL: http://example.com
    Username: John Doe

    user@host ~ $ pass --clip Website/Example
    Copied Website/Example to clipboard. Will clear in 45 seconds.

    user@host ~ $ pass --meta Website/Example
    URL: http://example.com
    Username: John Doe

    user@host ~ $ pass --clip --meta Website/Example
    Copied Website/Example to clipboard. Will clear in 45 seconds.
    URL: http://example.com
    Username: John Doe