Add a config.str cast that "eval" shell strings (e.g. strip double/single quote)

[osantana]

Useful to make .env more "compatible" with a sourceable shell file. And avoid problems with things like silently fail on parsing this kind of URLs:

• DATABASE_URL="postgresql://user:pass@host/db"

Optional (depending on security concerns):

• User expand like ~ -> ${HOME} or ~user -> /home/user|/Users/user
• Expand at least environment variables (how useful this feature could be?)