Allow ExMachina.Ecto to be used without the :repo option

[col]

We should be able to use Ecto without a repo. This is useful when using ecto without ecto_sql.

ExMachina currently raising an ArgumentError when using ExMachina.Ecto without a repo. This change now allows the repo to be nil and raises a RuntimeError when the insert/1 function is called instead.

New error message should still be helpful enough for people that simply forgot to add the :repo option.

insert/1 is not available unless you provide the :repo option. Example:

use ExMachina.Ecto, repo: MyApp.Repo

I believe this addresses the following issues:
#369
#355

[germsvel]

@col thanks so much for opening up this PR! It looks really good.

I saw you added some tests for when we raise the error. Would you also be willing to add a couple of tests that ensure params_for and string_params_for work without the repo being passed in?

[col]

Sure will do.

[col]

@germsvel - Done. I kept the tests pretty basic. Let me know if you'd like the tests to cover anything else more specifically. Looks like the repo is only used by the ExMachina.EctoStrategy which only affects the insert functions.

[germsvel]

Thanks @col! I gave it a quick glance, and things look good. I'll give it a better look when I have a few more minutes, but I think this is probably good to merge.

[germsvel]

This looks good! Thanks for the PR @col! 🎉

[col]

This looks good! Thanks for the PR @col! 🎉

You're welcome. Thanks for merging. 👍