Add option to run vignettes on Binder

[martintoreilly]

MyBinder supports running R Markdown files in a remote hosted RStudio environment (via the "New" -> "Rstudio session" menu option in the Jupyter filelist view). It would be great to have a "Launch Binder" button in a "Try out the package" section of the README, along with some instructions on how to launch the RStudio session and run the vignettes.

Notes

When running the NetEMD example vignette on MyBinder, it complains about the package phangorn not being installed. This is currently in the "Suggests" section of the DESCRIPTION file. Moving this to the "Imports" section would allow the example vignette to be run without the user having to manually install phangorn. We should check all the example vignettes can be run with no manual package installs as part of adding the "Launch Binder" option.