PushBullet - Configure premium status when required

[iffy-pi]

This is just an improvement to the user experience. Specification:

• Users are only asked about their premium status when the file being pushed is over the 25 MB size limit
• If the user has not configured their premium status yet, they are presented with a menu asking if they have paid for PushBullet premium. Their choice will be saved into the config settings and will determine if the shortcut continues
• If the user's configured premium status is True, then don't show any menus
• If the user's configured premium status is false, prompt them about the error and give them the option to either say they have pushbullet premium or no.

[iffy-pi]

Tested it and it works to specification, will be part of PushBullet release #68