Add support for multiple Accounts/API keys

Because scalyr/dataset only supports making API keys per-account, and this app only supports saving one API key, if you're managing multiple accounts you effectively have to pick one account to use with the app at a time.

A possible solution to this would be to define the API/URL information for an "Account" then define the "Account" in the search command, for example:

[XDR-dev]
URL = <dev url>
Read API = <dev Read API token>
Write API = <dev Write API token>

[XDR-prd]
URL = <prd url>
Read API = <prd Read API token>
Write API = <prd Write API token>

The search command could then specify which account to use:

| dataset account="XDR-dev" method=powerquery search="<powerquery>"