Support reading database connection string from environment variable

[chiawendt]

Is your feature request related to a problem? Please describe.

Connection strings with credentials must currently be passed via CLI args, which can leak through shell history, CI logs, crash dumps, or ps listings.

Describe the solution you'd like

Allow migrate to read the connection string in this order:

1. -database arg
2. Env var specified by -env flag (default: DATABASE_URL)

[AlexHill]

Edit: I just needed to add an extra slash to the DSN - postgresql:/// rather than postgresql:// allows me to specify everything using environment variables.

I would love this too, in a slight variant - supporting the standard libpq environment variables for the PostgreSQL backend as documented here.

This would allow us to specify an empty DSN like postgresql:// and have the appropriate paramters configured via the standard env vars. It would also make the password less sensitive to non-ASCII characters, etc.