Dedicated caplog recording handler

[abusalimov]

Changes affecting users:

• Now caplog.text always uses the same formatting, regardless the --log-format option. You won't want your tests fail because the user prefers another formatting, after all
• Changing the logging level through caplog does not affect captured logs shown in case of a failure (this is still not the case when using caplog.at_level/set_level(logger=...) though, but I'm going to address this one too in future)

Note: This contains changes that you might happen to review as part of #7, I decided to drop these changes from there eventually.