Environment variables support for commands

[saheljalal]

For each Command it would be interesting to add support for a map of env vars that can be included as part of the configuration. Similar to how task does it, for additional functionality to improve command trees which can inherit them.

Additionally, sourcing .env files and variants as an option can be useful as well.

Often times workflows also requires sourcing scripts but what if the scripts are distributed in different repositories or locations. Since we have the ability to download from many data sources (thanks to go-getter) we can use the same mechanism to download and sync various files to source for a command tree. That means any shell commands added to a node become available to all commands in the tree and is automatically sourced on evaluation.