Add default tag mappings

[jsumners]

This PR adds default mappings for tags to log levels. This reduces the amount of configuration necessary to use the plugin in a standard logging style.

[mcollina]

👍 ! Good job.

Can you add this to the README?

I think we should also expose the default tags on a property of the module itself, so it's easier to extend.

[davidmarkclements]

on that shouldn't this be something like

Object.assign({}, defaultTags, options.tags)

So that if a user supplies {trace: 'foo'} then debug is still 'debug' etc.

[jsumners]

@davidmarkclements well, the current implementation is an all or nothing situation. I simply replicated that. Do you guys want the supplied option to extend or replace the defaults?

[davidmarkclements]

..not sure - @mcollina ?

[mcollina]

I'd say let's extend the defaults :). Those are sound!

[jsumners]

Okay, the defaults are now extended, the mappings exposed, and the documentation updated.