Offer connections to env vars

[kris-steinhoff]

Look for env vars that start with DATABASE_URL_ or end with _DATABASE_URL. Add option to connection prompt to use those values. For example:

$ dbcl
Found several DATABASE_URLs:
1. DATABASE_URL (mysql://...)
2. APP_DATABASE_URL (mysql://...)
3. WAREHOUSE_DATABASE_URL (postgres://...)

Connect to [ ]:

If the input at this prompt is an integer, use the corresponding URL to connect.