add new and shift triggers

I'm working on getting the select2 field type working in a repeat group. in order to do so, i needed these triggers. 

note: some of the background / solution w/ regards to select2 can be found here:

https://github.com/mustardBees/cmb-field-select2/issues/10

once these are merged, i'll do a PR for the select2 fixes as well. 

p.s. I read somewhere that WP was going to (for lack of a better word) embrace select2 in a future release. perhaps it would make sense for CMB2 to have select2 as native sooner rather than later? just a thought.