Add options `overridesDist` and `swaggerUiDist`

Add an option `uiDirs`

The `uiDirs` option allows users to provide their own set of directories
with UI files, e.g. to provide a custom swagger-ui fork and a custom
set of style/font overrides:

    explorer(app, {
      uiDirs: [
        path.resolve(__dirname, 'public'),
        path.resolve(__dirname, 'node_modules', 'swagger-ui')
      ]
     });

The existing option `swaggerDistRoot` is deprecated now.

Close #66.

/to @raymondfeng please review
/to @ritch @STRML the option names are not great, can you advise better names?
/cc @shelbys