rstudio not referenced as a dependency

[stevenolen]

https://github.com/mobilizingcs/MobilizeSimple/blob/master/R/load_lab.R#L62

explicitly uses the rstudio package without a fallback. needs to be added as a dependency

[stevenolen]

This one may not be as simple. probably needs a fallback when running in a CLI to at least say rstudio gui is required for this interaction.

> install.packages('rstudio')
Installing package into ‘/usr/local/lib/R/site-library’
(as ‘lib’ is unspecified)
Warning message:
package ‘rstudio’ is not available (for R version 3.2.0)