add remote mixins

[kindermax]

To be able to use shared lets configs we could introduce remote mixins

PoC would look like this

mixins:
 - lets.test.yaml
 - url: https://some.path.com/lets.lint.yaml
   version: v1

This will download this mixin in .lets dir and cache it

Thing to consider:

• how to version remote url ?
• do we need to use latest ?
• how to force download or cleanup remote mixins ?
• what if network is unavailable ? how long to wait ?
• terminal progress UI for downloading (--upgrade command would benefit from it too)

[kindermax]

mvp is in 49 version. Still a lot todo