Add ability to pull/push under different goth service accounts.

[noizu]

I have a requirement on this functionality for synchronizing my stage/dev environments with the production data sets. It may also be useful if you have a restricted auth account by default but sometimes need to use an elevated account to make certain requests.

The feature works by pushing a value to the current process dictionary and referencing that value when preparing request tokens or determining the current project_id.

If the functionality is not referenced I've constructed the Goth calls to use the previous format (rather than just setting account to :default and using the {account, field} version of methods.) to avoid impacting any upstream users if they, for example, have overridden the module used by defp token_module() and their implementation does not support the for_scope({account, scope}) format.

The Readme has been updated with an example of how to use this feature:

require Diplomat
# copy data from prod to stage environment
# 1. Fetch data from production account
prod_data = Diplomat.with_account(alternative_account["client_email"]) do
  Diplomat.Query.new(
    "select * from `Book` where name = @name",
    %{name: "20,000 Leagues Under The Sea"}
  ) |> Diplomat.Query.execute
end
# 2. Write to stage/dev account (default environment)
Enum.map(prod_data, fn(entity) -> 
  Diplomat.Entity.upsert(entity)
end)

[noizu]

o_O