Add sane logging

[moigagoo]

Currently, the only way a preprocessor can signal about a problem is by crashing the build. Also, there may be preprocessors that do not modify the source but rather check it producing diagnostics information, e.g. linters. Such preprocessors have no standard means to yield their output.

I propose Foliant logs build activities in log files. Multiple variants can be implemented:

• a single log that's constantly appended with new data
• per build logs
• per preprocessor and per backend logs

[moigagoo]

Done in 1.0.4.

[moigagoo]

Done in 1.0.4.