Make log filterable

[linawolf]

Feature request

If the log is very big, it is hard to work on. Make it filterable for easier handling:

• Errors only
• Parsing only

Furthermore warnings during rendering are usually thrown once for each handler, that would be 3 times in total for TYPO3

[jaapio]

Suggestion to implement this using multiple loggers, one per stage. This will allow us to provide filters and other monolog features to make the output of this library configurable.