Add `skip_missing` keyword to `populate!`

[rofinn]

In some cases you may want to fill a pre-allocated KeyedArray with table data and have it ignore key values that aren't in your pre-allocated array. This might be a nice way to gracefully avoid erroring here when the key indices aren't present?

[mcabbott]

Sounds good. It already takes on keyword force=false to disable over-writing, so maybe skip=false is the default too (all the data must end up in the container) while true keeps a subset.

(I guess subset and intersect are other words which could make some sense, but perhaps avoiding the suggestion that ::Missing is involved is good.)