Add inline option for RadioButton and checkboxGroupInput

I was looking for horizontal inline placement for choices in `radioButtons` and `checkboxGroupInput` when the number of choices are so small that they waste a lot of space if aligned vertically.

I came across this `gsub` solution and can confirm it works.
http://stackoverflow.com/questions/17277210/r-shiny-side-by-side-checkbox

I'm wondering if it's desirable to have an `inline` option in `radioButtons` and `checkboxGroupInput` to have `radio` and `checkbox` inlined from the get-go. Let me know if anyone knows better solutions.