Make it possible to give cmdline a directory to extract attachments to

[petri]

This is mostly motivated by testing; it's rather annoying to always find the extracted files in the current directory.

Gonna reshuffle the cmdline params a bit for this; will use -o | --output as the param and make overview use -c | --contents from now on.

See tnefparse --help for revised usage.

[petri]

Bah forgot -c was used for checksum. Keeping -o as is, using -p | --path for explicit output dir instead.