Custom console log formatters

[zachleat]

Allow people to customize the default output that Eleventy logs to console.

This could be used to log the gzip compressed size of a template, a la:

[zachleat]

A vote to disable [11ty] prefix for folks that already have a line prefix: https://mas.to/@jamesdoc/115745634195806806

[jamesdoc]

Thanks for pulling this in Zach - for future reference this is coming in thanks to concurrently.