Implement `eventgrid` command

[marstr]

The eventgrid command extends BuffaloCommand:"generate".

When it is executed it should:

1. Update the app.go file to include an action for receiving Event Grid events.
2. Add a file which will contain methods to be implemented by the Buffalo user. a. Without specifying event type information, there should be a single handler registered to "all" which receives an eventgrid.Event. Signature: func WellChosenName(c buffalo.Context, e eventgrid.Event) error b. When type information is specified, a handler for each event type should be generated. A type should also be generated which encompasses the schema of the Event given to it. Example:

   
   type IntEvent struct {
   eventgrid.Event `json:""`
   Data int `json:"data"`
   }

func AnotherWellChosenName(c buffalo.Context, e IntEvent) error { // ... }

3. [stretch, may be incorporated as a separate issue] If an Azure deployment template exists and a Topic ID is provided, update the template to subscribe this Buffalo App to the Topic.

[Buffalo Directory Structure for Reference](https://gobuffalo.io/en/docs/directory-structure)

edit: The strategy of generating a bunch of types was both harder for me, and not really helpful for users. Instead, `eventgrid.Event` has a `json.RawMessage` to allow partial unmarshaling by the event, and further unmarshaling by users.

[marstr]

Here are some examples of the composition scheme above being processed by the JSON encoder:

• https://play.golang.org/p/Um4qGsvI3Pl
• https://play.golang.org/p/4iqkH2Kpgdv

edit: the examples work, but are no longer what we are considering.

[marstr]

Before this issue is closed, #17 and #18 should be completed.

[marstr]

Done 😃