Implement the Limits API

[arcanericky]

The Pushover Limits API is simple and has a very short writeup at https://pushover.net/api#limits. It uses only a single parameter (token) and can be viewed with:

$ curl https://api.pushover.net/1/apps/limits.json?token=apitoken
{"limit":7500,"remaining":7405,"reset":1564635600,"status":1,"request":"7e3888b4-0bc8-4525-b567-d30e11b68d92"}

Implement this API in a limits.go module, including appropriate comments for GoDoc and unit tests. Use the Messages and Validate API as examples.

Use this API in the Pushover CLI utility with a limits commmand. An example might be:

$ pushover limits --token apitoken
Request
Token:       apitoken

Response
HTML Status:      200 OK
HTML Status Code: 200
API Status:       1
Request ID:       7e3888b4-0bc8-4525-b567-d30e11b68d92
Limit:            7500
Remaining:        7405
Reset:            1564635600
Response Body: {"status":1,"group":0,"devices":["pixel2xl"],"licenses":["Android"],"request":"a5669be9-1160-40cb-b680-36dde650bc28"}

Include unit tests for the CLI functionality.