feat(chips): with autocomplete should allow multiple selection

[csvan]

Currently, it is only possible to select a single chip in an md-chips instance at a time. It would be useful if several chips could be marked as selected as well, akin to how this is handled in md-select when the multiple attribute is set.

[jaymanned]

I absolutely miss this feature. Any workaround?

[jaymanned]

Of course we would need a way to know which chips have been selected. And it would be great if it worked also on static chips (which I simulate by providing an empty md-chip-remove element)