Read from `$DATABASE_URL` automatically

[musjj]

description

The DATABASE_URL environment variable is a popular convention for storing database urls. So it'd be nice if rainfrog can automatically pick it up. Even better if it can automatically read it from the project's .env file.

For example, sqlx-cli provides a similar behavior:

This can be done either with the --database-url command line option or by setting DATABASE_URL, either in the environment or in a .env file in the current working directory.

proposed solution

Read from DATABASE_URL env variable automatically.