Add auth support via ENV

[jimaek]

To support our upcoming API keys we need a few changes:

• New parameter -T/--token (open to different options) - in Add auth support via command #108
• ENV VAR support to allow the user to set it on a shell or script level. GLOBALPING_TOKEN
• New command limits that will output IP limits if the user is anon and remaining credits if the user is authenticated - in Add limits command #109

[MartinKolarik]

@radulucut let's start by adding support for GLOBALPING_TOKEN variable, and I'll specify the rest of the interface to add a little later. The token is sent as Authorization: Bearer YOUR-TOKEN (docs). You can ping me by Slack/mail, and I'll send you some actual tokens for testing.