Add report log output at end of plugins command

[thrau]

Currently the log output of the plugins command is a bit of a mess. There's no real good way of seeing the warnings that some files weren't importable and scannable for plugins. A nicely formatted report at the end of the command would be useful, containing

• all discovered plugins and their files
• all files that had an error while importing (and the related error)

It would also be useful to configure the loglevel and capture all the output created by simply importing code, similar to pytests' --log-cli-level