Add support for changing severity using bang methods.

[ioquatix]

This is a proposal to make it easier for 3rd party implementations to support same interface without depending on Logger constants.

Rather than writing logger.level = Logger::DEBUG, we can write logger.debug!. I believe this is better since it is aligned with the "Ask, don't tell" software design pattern.

[sonots]

I like this. I am going to merge.

[ioquatix]

Awesome. Do you mind waiting for me to add more documentation?

[ioquatix]

I will commit documentation directly to master.

[sonots-zozo]

it is okay for me, thank you for your documentation.