Add support for external environment variables

[vitorsss]

Is your feature request related to a problem? Please describe.
I need a way to type a 'secret' or password without saving it in a .tape file.

Describe the solution you'd like
Add support to Type from an enviroment variable, so that i can use envchain or something similar to handle this secret externally, without exposing it's content.

Type $SOME_ENV_VAR

Describe alternatives you've considered
None.

PS. Sorry, I didn´t see the org's contribution guidelines earlier, and had already opened an issue and a PR about this.

[lucafmarques]

I think supporting external ENVs would be pretty good.