Add --warnings cli option; --silent now prints only errors

[aleksandarristic]

Changes:

• --silent now prints only errors. This is done by setting the log level to logging.ERROR by default.
• --warnings option added to cli.py; setting this argument will set log level to logging.WARNING.
• --warnings option also read from config file

[codecov[bot]]

Codecov Report

Base: 59.13% // Head: 59.13% // No change to project coverage :thumbsup:

Coverage data is based on head (a44ca30) compared to base (375aaf0). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #383 +/- ## ======================================= Coverage 59.13% 59.13% ======================================= Files 10 10 Lines 1336 1336 ======================================= Hits 790 790 Misses 546 546 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=domainaware). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=domainaware)

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