Feature request: add .env file support

[zroadhouse]

This is a feature request to add .env file support to xc as an alternative to adding Env statements at the task level. This would allow the same set of tasks to be easily used for different environments (dev, prod, etc) and keep the markdown as clean and readable as possible.

The format for .env would be the same as for other tools:

VARIABLE1=VALUE1
VARIABLE2="VALUE2"

The env file location could be specified using -env-file:

  -env-file <string>
        Specify a file containing environment variables (default: ".env").