API docs generation should not rely on a specified project structure

[iawia002]

现在我们很多 project 的目录结构都是自己定义的比较随意的那种，nirvana api 检测不到任何 descriptor

现在的想法是能不能在 run server 的时候再来生成 api docs，那个时候是已经拿到了所有 descriptor 的，也少了 nirvana api 检测/收集 descriptor 的步骤，把这个做成一个 Configurer，需要的话可以配置在启动 server 的时候生成文档或者直接把文档页面自动注册到当前 server 里，可以直接访问，比如：

$ go run main.go

INFO  0830-17:59:34.608+08 config.go:309 | Listening on :8080
INFO  0830-17:59:35.126+08 nirvana.go:231 | Generated openapi schemes to /Users/mac/iawia002/go/src/github.com/caicloud/nirvana/examples/api-basic
INFO  0830-17:59:35.127+08 builder.go:231 | Definitions: 1 Middlewares: 0 Path: /api.v1.json
INFO  0830-17:59:35.127+08 builder.go:246 |   Method: Get Consumes: [] Produces: [application/json]
INFO  0830-17:59:35.127+08 builder.go:231 | Definitions: 1 Middlewares: 0 Path: /api.v2.json
INFO  0830-17:59:35.127+08 builder.go:246 |   Method: Get Consumes: [] Produces: [application/json]
INFO  0830-17:59:35.127+08 builder.go:231 | Definitions: 1 Middlewares: 0 Path: /docs
INFO  0830-17:59:35.127+08 builder.go:246 |   Method: Get Consumes: [] Produces: [text/html]
INFO  0830-17:59:35.127+08 builder.go:231 | Definitions: 1 Middlewares: 0 Path: /api/v1/applications
INFO  0830-17:59:35.127+08 builder.go:246 |   Method: Create Consumes: [application/json] Produces: [application/json]
INFO  0830-17:59:35.127+08 builder.go:231 | Definitions: 1 Middlewares: 0 Path: /api/v2/applications
INFO  0830-17:59:35.127+08 builder.go:246 |   Method: Create Consumes: [application/json] Produces: [application/json]

/docs 就是自动生成的文档的网页版

/cc @kdada @ddysher @supereagle @bbbmj

[supereagle]

SGTM

[ddysher]

LGTM

BTW, internally, we should propose a semi-standard structure..