Add Passthrough, Progress indicator debug messages

[giachello]

Added the following

• Passthrough mode (-Y). This allows you to split the file: matched messages go to the a file (-o), non matched messages go to stdout.
• Progress indicator (-x). This prints to stderr the number of matched and processed messages, helpful with large mbox's
• Some additional debug messages

[dspiljar]

Here we have another huge, difficult to follow, PR which covers and introduces three or more functions at the same time. To the contrary to my polite request in #3, to create a separate PR for each new function.

To summarize:

• CRLF: Will not be merged.
• Passthrough: I see this as a duplication of existing functionality. Will not be merged.
• Faster mbox processing: Good idea, please create a separate PR.
• Progress indicator debug messages: Good idea, please create a separate PR.

Regards, Daniel

[giachello]

Daniel, I think you can probably split the PR because the individual improvements are in separate commits. I won't have time for this until later. Will close this one.