Support for default values in .env

[ilansh]

Would it be possible to support syntax of default values in .env files?
As in

DB_HOST=${DB_HOST:-127.0.0.1:3306}

This can prove very helpful for my usage pattern.

[ashald]

This will not be compatible with the underlying .env file spec, to the best of my knowledge, but I'm considering adding an option to allow variables expansion. Please follow #16 for updates.