Add support for local filters

[brandon93s]

This PR adds support for the following syntax:

import {moment, duration} from 'vue-moment'

export default {
   data(){ },
   filters: {
      moment,
      duration
   }
}

This allows the filters to be used without having to register them globally.

These changes are backwards compatible and will not impact current users.

[brandon93s]

@brockpetrie @BrockReece - Is there interest in adding this support? I'm currently using a local fork but would love to use vue-moment!

[BrockReece]

Hi @brandon93s.

Thanks for your contribution, the change looks good on the surface, I will have a closer look tomorrow though.

[ahoiroman]

Any chance to get this into the package?