Add utility package logsource for different ways to read postfix logs

Ref #82 (closed)

Refactor the interface for the diverse ways of obtaining postfix logs:

• via text files (or stdin)
• via a directory

It also makes it easier to implement new source log sources, such as logstash or journald.

This refactoring was extracted from the ongoing work on the mail log tracking (#82 (closed)) and in general improves the code quality.