Allowing multiple accounts

[minsis]

It would be nice if this pack could support multiple accounts similar to the email pack. Although this would probably introduce a breaking change unless maybe an optional array is created.

Allowing multiple accounts:
https://github.com/StackStorm-Exchange/stackstorm-email/blob/master/config.schema.yaml#L4

When running something you specify the account you want to use:
https://github.com/StackStorm-Exchange/stackstorm-email/blob/master/actions/send_email.yaml#L28