This PR rewrites most of the app's core functionality to allow to extend it in the future if needed. Most of the behaviour has been kept, except for some new handling. Namely, the deprecation of --stdin (which was quite short-lived) in favour of --file - to keep it in line with other tools like kubectl and better handling of environment variables file which might break old laxed versions running in the wild.

The new addition is also the support for a custom Helm-like values file: a values file like this one:

name: Patrick

Can be accessed from the template:

Hello! My name is {{ .name }}

But to also maintain consistency with other tools like Helm, you can access it through .Values.name as well:

Hello! My name is {{ .Values.name }}

With the rewrite, I've also added a plethora of tests for some edge cases some users reported privately. One of these cases was how I handled the environment variables file. It should be more clear and somewhat close to static strings in Bash, although without support for string interpolation.

Due to the removal of the --stdin flag, the new version cut will be a major version change.