change to possibly use RStudio viewer if available;  currently not helpful

This is currently not helpful since the [html/svg is not responsive](https://github.com/bryanhanson/exCon/blob/master/inst/extdata/eC_globals.js#L6-L24).  I probably should have worked on this in reverse order, but this change is much easier to implement.

Another change is to not use `setwd()` since it is not necessary and could potentially cause problems with things like `shiny`.

I did not see any tests, but my manual tests passed in my Windows environment.

Another note if you are not aware is `options(viewer=NULL)` turns off the RStudio viewer.