add settings

[kindermax]

Idea to add settings to config.

It then used as LETS_SETTING_<NAME_OF_SETTING>

Settings can be stored in .lets/settings.json

Add new --set and --reset flags.

Example:

settings:
  skip-pull: false

So LETS_SETTING_SKIP_PULL=false will be available in env.

lets --set skip-pull=true will override and persist setting to .lets/settings.json