Support skipping logger creation if it is provided

[potatopankakes]

The allows:

• an app to provide their own logger.
• skip the creation of the winston logger.
• avoid constructing winston transport configuration, when not used.

[msluther]

Seems like a fair enough feature.

The docs could use some updating though to reflect this.
e.g. https://github.com/godaddy/slay#app , https://github.com/godaddy/slay#app-startup-in-detail
And perhaps a sample of how to provide your own logger and what that "interface" for a logger should look like (given that some internal components to slay are going to call methods on app.log).

Also perhaps an e2e test showing that a custom logger can work here without crashing, stuff actually gets logged to it, etc.

[msluther]

I'm not super familiar with gulp, but depending on platform and how this gets used you may need to include both test/*.js and test/**/*.js.

[indexzero]

Reasonable enough indeed, but we should probably put in the effort to discuss the capabilities of such a module an document them in-depth before saying "any object will do for app.log".

A good way to start that discussion is: what logger are you replacing winston with? We can do a capabilities diff from there as a straw person.

[indexzero]

This capability checking is not quite enough as there are a few calls to app.log.info in the other preboots.