Add support for custom filtering in Shiny app

[jwildfire]

I think filtering will be a really useful feature. A few thoughts came to mind for improvement: 1. allowing "OR" between variables 2. use the RStudio "subset dataframe" add-in idea to allow query strings 3. providing a slider bar filter for continuous variables (via Xiao Ni)

[jwildfire]

This would be tricky to support in the JS repo, but would be an excellent addition to the r pacakge