feature request: add min-limit prop to subslot

[pretzelhammer]

Currently there's a limit prop on subslot which allows us to set a maximum limit of components used in the slot however I would like to also have a min-limit prop which would allow me to set the minimum amount of requires components to be used in the slot. I would expect some event to be emitted if this minimum requirement is not reached.

Speaking of emitted events, currently subslot only supports no-match and I propose mismatch here but I think all of these could be consolidated into a single validation-error event which returns an object describing all validation errors, e.g.

{
    errors: ["noMatch", "mismatch", "overLimit", "underLimit"]
}

[privatenumber]

This might be worth adding to the non-mixin API, but if you use the mixin API, you should be able to check against $subslots[subslotName].length on the updated life-cycle hook