Add option to read swagger spec directly from URL

[Tradunsky]

Springfox swagger does generation of swagger.json at runtime, because of that we need to run app to get that file and refresh DapperDox. With this simple param, DapperDox will receive fresh swagger spec directly from the service running with springfox.

[Tradunsky]

ok, first build has failed because of running with master version of spec.go and config.go, but main.go is from serve-spec-by-url. So, I decided to separate main.go part into separate PR after this one is merged.

as about this build, I've got confused.. not sure what is the root cause of such error, need to investigate.

[Tradunsky]

@zxchris Please take a look

[frinka]

@Tradunsky How is this different from providing a URL to the spec-filename parameter? For instance, we use -spec-filename=https://our-app.oursite.com/v2/api-docs, which is where the swagger.json is hosted by springfox. You would still need to restart DapperDox either way to get the latest the version, right?

[Tradunsky]

@frinka Sorry for late response.
ok, I didn't know that in spec-filename you can specify an URL to resource, moreover, according to doc, you also would need to specify spec-dir where that files are located.
Are you sure that URL is supported? If so, it looks a bit confusing. Not sure if even correcting doc can help in this design approach.