Capture logger output

[jonatanklosko]

Closes #136.

Adds a logger backend to the evaluation node, which backend logs messages to sender's group leader iff it's an instance of Livebook.Evaluator.IOProxy.

This is essentially a stripped down version of Logger.Backends.Console. It reads basic configuration like log level and format from the same configuration, which may be handy when running inside a mix project, but let me know if it doesn't make sense.

Also, is there a good way to test the backend in isolation?

[josevalim]

I discussed adding this to Elixir itself but given Elixir plans to eventually migrate away from Logger.Backend.Console and use Erlang's handler, we would need to contribute this to Erlang handler first and that seems to be less trivial, so 👍 to getting this in. I will review the PR shortly.