Add limits command

[MartinKolarik]

Must work with both persistent login (#108) and ENV var tokens. See jsdelivr/globalping#328 for the API endpoint and format (not live yet).

Authenticated user (get the username from the introspection endpoint):

$ globalping limits
Authentication: token (jimaek)

Creating measurements: 
 - 500 tests per hour
 - 150 consumed, 350 remaining
 - resets in 22 minutes

Reading measurements: 
 - 25000 requests per hour
 - 14650 consumed, 10350 remaining
 - resets in 22 minutes

Anonymous users:

$ globalping limits
Authentication: IP address

Creating measurements: 
 - 500 tests per hour
 - 150 consumed, 350 remaining
 - resets in 22 minutes

Credits:
 - 10480 credits remaining (may be used to create measurements above the hourly limits)

[MartinKolarik]

The endpoint is now live: https://api.globalping.io/v1/limits. Docs: https://www.jsdelivr.com/docs/api.globalping.io#get-/v1/limits

[radulucut]

@MartinKolarik reading limits are missing from the response. Also, the credits value is for authenticated users only, right?

[MartinKolarik]

They will be added later in the same format, consider them optional. Credits only for auth, yes.