add :deleted column to any schema that uses alog

As a developer creating Phoenix Web Apps/APIs using **`alog`**,
I would like to have the **`:deleted`** column added to each schema/table where **`alog`** is used.
So that I don't have to _manually_ add the **`:deleted`** column
or _think_ about how to "delete" records.

# Todo
+ [ ] automatically add a **`:deleted`** field to a schema when the **`use AppendOnlyLog`** is employed.
+ [ ] Write tests for schema/field creation
+ [ ] Document this in the `README.md` so it's clear to people _why_ this column is needed/added.

as initially discussed in https://github.com/dwyl/phoenix-ecto-append-only-log-example/issues/7#issuecomment-431863209