Improve format autodetection

[rr-]

Right now the program just loops through all possible format readers and tries to unpack with each one of them. Exception happened? Try another reader.

This causes ugly anomalies. For example:

• I see XP3, and I know for sure it's XP3 thanks to XP3\r\nWhatever magic number inside. But I haven't supplied --plugin, which causes me to throw an exception. The file is mistakenly delivered to next reader.
• I see ANM archive, and it's nice. It correctly unpacks 0 files. But in reality it was a NWA sound file all along, I just didn't happen to know this because ANM was earlier on my check list.

This should be fixed. I could add new exception, say, RecognitionError and catch only that, but this won't make any good to detecting false positives (example from second point).

What should be done instead:

• Add to every Transformer (Add common interface for archive and converter #14) a new method, is_recognized
• Before calling unpack, loop through all Transformers
• See how many of them recognize the file
  • If it's just 1 - great! Call unpack on it, and then call it a day.
    • The unpacker now throws an exception urging you to provide --plugin parameter. Everything goes well, because the file is no longer passed to the next transformer.
  • If it's more than 1 - this is good, too. I can tell it to the user and advise him to select the --fmt manually.
  • If it's 0, tough luck. File is not recognized.

Cons of this solution are limited and, in my opinion, negligible:

• no account for situation when is_recognized needs --plugin to be able to recognize the file perfectly. In these cases we can try looking at file extensions, their names, etc.
• I need to implement is_recognized for every support format. This is going to be tough!