Refactoring and fixes to Swagger Middleware

[gaby]

• Added support for using YAML as Swagger spec. Fixes 🤗 [Question]: swagger yaml support? #708
• Migrated from fasthttpadapter to fiber/v2/adaptor
• Added support for Next(), and other missing checks
• Update documentation
• Added support for Path and Title
• Fixed bug where the middleware was reading the Spec and assuming JSON then passing this as Bytes to the go-openapi middleware.
  • Instead now we have a http.HandlerFunc able to serve the SPEC based on extension type back to the middleware
• Added support for running multiple SwaggerUI's, for example:

// Create Swagger middleware for v1
//
// Swagger will be available at: /api/v1/docs
app.Use(swagger.New(swagger.Config{
    BasePath: "/api/v1/",
    FilePath: "./docs/v1/swagger.json",
    Path:     "docs",
}))

// Create Swagger middleware for v2
//
// Swagger will be available at: /api/v2/docs
app.Use(swagger.New(swagger.Config{
    BasePath: "/api/v2/",
    FilePath: "./docs/v2/swagger.json",
    Path:     "docs",
}))

• Added extensive unit-tests to verify normal handlers still work.
• Replaced join.Path with url.JoinPath
• Use testify.Require() instead of testify.Assert()
• Added documentation to swagger.Config{}
• Added example swagger.yaml

Note: I don't consider this PR a breaking change, since the original functionality stayed the same, it's mostly bug fixes and new features.