[Feature Request] Allow adding custom spacings to vuetify utility class

[priosshrsth]

Problem to solve

I think eveyone sometimes need to use utility class like mt-22 or pa-22. Currently, only way is either creating own utility class or overritding $utilities to use new spacers.

Proposed solution

I think vuetify should check if the spacers has already been defined before assigning so we can use custom spacers.