Add user controlled case selection to shinyMer

[jknowles]

Currently shinyMer only allows the user to select a random or an average case. It does not let the user specify a specific subset to take a random or average case from, although within the functionality of draw this is easily accomplished.

This could be a reactive panel on the 3rd tab that appears if the user selects "User Specified Subset" and then it would kick off some server-side code to allow inputs for variables in the model frame and the values to use for subsetting. This could allow much more rich exploration of effects on the fly.

[carlbfrederick]

I think this is a good idea... perhaps with some wiggle functionality too to make the single observation cases more interesting to look at.