Added a packages= option to read_RStudio_CRAN_data

read_RStudio_CRAN_data fails on out-of-memory (in the rbindlist call) if you've downloaded many RStudio CRAN logs.  This patch adds a packages= option if you're only interested in a subset of packages, which subsets the daily log data on the fly.  Other such filters might also be added.
Merge pull request #7 from friendly/patch-1

Added a packages= option to read_RStudio_CRAN_data