Support env.local, et al. or support `--envfile` CLI option

[hparra]

dbmate does not support additional .env files, e.g. .env.local or .env.development.local due to https://github.com/joho/godotenv/issues/10

Possible solutions of varying completeness:

• Have loadDotEnv also load .env.local
• Add --envfile option which specifies file for godotenv.Load()
• Use a fork of godotenv which implements https://github.com/joho/godotenv/issues/10

[msdrigg]

I also would like this issue supported

[amacneil]

Happy to accept a PR for this, I think it would only be a few lines of code

[msdrigg]

I haven't written any go code, but if this is still sitting open in a week or two, I may give it a try.

Also, if anyone does write a pr, I would definitely prefer the solution of

Add --envfile option which specifies file for godotenv.Load()

[jahilliard]

I'd love this too.

[jahilliard]

Looks like something like godotenv.Load(".env.local") is supported.