Version.json: verify if gommitizen it's already initialized.

[sergiotejon]

In version/public we could add a method called, for example, is_initialized to verify if the file version.json exists in the folder and if it is a well-formed file.

It must return true or false.

Use this method in the function initialized of cmd/init to verify before saving.

[sergiotejon]

@jaimeph, is this solved already?