Migrate to go 1.16 embed

[xakep666]

Will close #5

This solution contains only two lines of go code (variable declarations). But copying dist from submodule is necessary because submodule will not be downloaded during go get or go mod download. It leads to compilation error for module users.

After merge of this PR dependend projects (http-swagger, echo-swagger, gin-swagger) may be easily refactored using http.StripPrefix(prefix, http.FileServer(http.FS(swaggerFiles.UI)))

[ubogdan]

We are aware of go 1.16 embed feature. Unfortunately, we can't go with this approach because the library won't be "backward compatible" for users building with older versions.