Support fallback values for env-variables

[matsn0w]

It would be nice if one could enter a default value to use when a given environment variable is not set.

For example:

services:
  redis:
    version: 7.0.0
    port: '${REDIS_PORT:-6379}'

The herd init command currently throws an error when, in this case: REDIS_PORT, is not set in the .env file:

❯ herd init

   INFO  Setting up your project…

   ERROR  Invalid herd.yml file…
   ERROR  The port for the service "redis" is required. Please check the .env key for: REDIS_PORT.