Adding rprofile argument to debian_install

[csgillespie]

Provide more flexibility to the Rprofile creation.

1. The default value should provide the same behaviour, so hopefully not breaking anything.
2. I did name the repo as CRAN which in theory might someone's set-up, but seems unlikely
3. Used shQuote() to hopefully avoid quoting chaos.

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Related to #202

[sckott]

thanks @csgillespie !

Might it make more sense to have rprofile parameter expect a named list? Then we'd change the default to list(repos=c(CRAN="https://cloud.r-project.org/")) and internally then coerce the named list into a character string. Does that seem like a better or worse option?

[csgillespie]

I thought about that, but the downside is we are limiting entries to options(). If we want to add other entries, say functions, then we can't

[sckott]

right. I guess then it's up to the user to make sure their string is formatted correctly

[csgillespie]

@sckott Are you happy to merge?