define and document default behavior

[hops]

As we add more functionality I'd like to have some clear defined default behavior.
The ones I have in mind right now are:

• Unless a file is specified it must read from stdin
• Unless a file is specified it must write to stdout
• Infos (e.g. stats) must always be written to stderr
• Input lines in the $HEX[] format must always be decoded before processing
• Output must always be encoded using the $HEX[] encoding if at least one character is outside of \x20 - \x7e

Of course there will be the occasional exception. For example formatting the output of the unhex tool in $HEX[] would be pointless as it's purpose is to decode such lines.

I'm open to ideas and suggestions, hence the discussion label.