ProjectionVersion: Add option to define a schema prefix

[sascha-wolf]

This PR adds the option to define a schema prefix for the ProjectionVersion
schema. It offers two ways of doing this:

1. via the config

config :commanded_ecto_projections,
  schema_prefix: "example_schema_prefix"

2. via the use clause

defmodule MyApp.ExampleProjector do
  use Commanded.Projections.Ecto,
    name: "example_projection",
    schema_prefix: "example_schema_prefix"
end

If none is provided is uses a nil prefix which is the default for Ecto.

[slashdotdash]

Great, thanks for taking the time to make this change @zeeker. I'll merge, bump the mix version, and publish to Hex.

[slashdotdash]

Released as v0.5.0.