Create a logger implementation

[igorschoester]

Describe the feature you'd like

A logger implementation for use in Yoast SEO.
At the moment we implement a NullLogger and that can be changed with the wpseo_logger filter.

I'm not sure what the logger should exactly do.

• Do we want to write to a text file?
• Do we want to show the message as a notice?
• Do we want both implementations and an option to pick which to change to in the test helper?

Then an interface should be made to turn this functionality on/off.