Allow serving static assets from baseDir

[davidjbng]

Resolves #65

This PR documents how to serve static swagger-ui files from a custom directory and fixes serving static assets when the baseDir option is set.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm

[Fdawgs]

Few issues with doc changes.
Will flesh this out once I'm finished at work.

[mcollina]

@Fdawgs ptal

[DavidTanner]

Any idea on when this can be merged?

[knolleary]

I'm concerned this is a breaking change in behaviour and needs to be highlighted as such when it gets published - but would like a second opinion.

Prior to this change, if you set baseDir, the swagger static content would be served from the static folder included with this module. You could then set baseDir to point at another directory of content that should served in addition.

With this change, if you set baseDir it expects you to have copied the static folder included with this module into your baseDir.

So a working configuration prior to this PR will no longer work when this PR is applied.