CLI tool to mimic envsubst

[pilosus]

Is your feature request related to a problem? Please describe

Using Piny as a CLI tool is painful. We should add CLI too feature to mimic gettext's envsubst tool.

Describe the solution you'd like

We should use click to build CLI tool. It should:

• be called piny
• mimic envsubst interface
• be able to work with pipelines, stdin/stdout, substitute envs in a file in place (envsubst cannot do it in place)