Added access token flow to CLI

[olliekm]

Implemented flow for authenticating users in CLI:

• Long lived token is read from config.json on CLI start.
• Stores token in AppContext.
• Added additonal confirmation to mist auth login.
• mist auth login now opens browser and waits for user to input token, we call API (mocked right now) for long lived token and store it in AppContext and update config.json.
• AppContext is binded to Kong to make accessible in all commands for future API calls.

Notes:

• Token is displayed to user when attempting to log in, maybe change this in the future for security reasons.
• authURL currently is placeholder, needs to be updated to login site.
• getLongLivedToken needs to call auth API.

[blobcode]

@WarrickT take a look if you get a chance - otherwise lgtm