Remove requirement for Rstudio

[annakrystalli]

Related to #11 and ropensci/dev_guide#183.

I wonder if we could give the option to easily do away with rstudio dependency by purling the index.Rmd into an .R script. @adamhsparks, would a testing workflow in a script be better for you?

[adamhsparks]

I'm pretty agnostic about it. I tend to use RStudio for most things anyway, but the way I see it flexibility is good so if you can do away with the dependency that's a nice bonus. It wasn't an issue that I had to use RStudio, it was an issue that this wasn't documented.

[annakrystalli]

Thanks for the feedback @adamhsparks. I'll keep this as an enhancement issue but perhaps not a major priority.