ft: add Peep storage

[hauleth]

Change description

Adds peep as a possible storage backend.

Example usage

use PromEx,
  otp_app: :my_project,
  store: PromEx.Storage.Peep

Additional details and screenshots

Requires #240 before this one can be merged.

Checklist

• I have added unit tests to cover my changes.
• I have added documentation to cover my changes.
• My changes have passed unit tests and have been tested E2E in an example project.

[coveralls]

coverage: 78.217% (-0.08%) from 78.292%
when pulling 814cc24 on hauleth:ft/add-peep-storage
into 6022d47 on akoutmos:master.