Prompt user for variables

With the use of uservariables, one can define specific variables that need to be initially filled in manually by the user on first run.

The provided values are then automatically saved by dotdrop to uservariables.yaml, which can be included in the main config as a file from which variables are imported using import_variables.

Let's say, for example, that you want to manually provide the email value on new hosts you deploy your dotfiles to.

You'd add the following elements to your config:

  emailvar: "email"
    - uservariables.yaml:optional

On first run, the emailvar is prompted to the user and then saved to uservariables.yaml. Since this file is imported, the value for emailvar will automatically be filled in without prompting the user on subsequent calls.