generalize env names

[lopezator]

In order to open have a naming that makes sense when using different email providers, the proposal is to rename the following environment names from -> to:

MG_DOMAIN -> API_DOMAIN
MG_KEY -> API_KEY

So in #7 I can reuse them without incurring in a naming conflict.

We can keep this open until #7 comes, but wanted to open the issue to grab some feedback from you.

[haydenwoodhead]

I'm not sure about generalizing these environment variables. Not every email provider uses the same amount of environment variables. Sendgrid, for example, only uses one API key, or what about a hypothetical provider that needs four? I think it makes more sense to just have provider-specific variables. Then, if you aren't using that provider don't include them. This is the current strategy for databases (DATABASE_URL vs DYNAMO_TABLE).

My suggestion is to just create a new env var SG_KEY or SENDGRID_KEY. Thoughts?

[lopezator]

That makes sense. Closing.