Instantate multiple receivers by given config

config.accounts[].receivers allows users to define multiple receiver
instances, not only one Stream receiver per account.

By default (when receiver definitions not given) manager will
instantiate stream receiver.

= Example configuration

``` yaml
accounts:
  account_name:
    token: XXX
    secret: XXX

    receivers:
      - stream
      - other_receiver:
          give: some/configuration
```
[refactor] Extract to Receivers.find
[refactor] extract to methods
[refactor] extract to Akane::Storages.find

Related-To: 5644ecc0720cf2e0f506bf91bceedff24952a10d
Merge pull request #2 from sorah/user-definable-receivers

Multiple receivers