Missing Documentation for `opts.logger` option

[iamjochem]

hi there,

your lib makes it possible to pass in a pre-existing bunyan instance (which will be used in preference of creating a new bunyan instance if given), see here:

https://github.com/villadora/express-bunyan-logger/blob/master/index.js#L28

... but this does not seem to be documented.

PS: you might want to check that opts.logger is, if swet, actually an instance of bunyan before using it.