fix keywords and wordchars in filtetypes.r

Currently `filetypes.r` has a rather random collection of keywords for R files. 

This commit fixes the primary keywords (as used in RStudio, an IDE specialized for R), and also adds the appropriate list of built-in constants. See https://github.com/rstudio/rstudio/blob/master/src/gwt/acesupport/acemode/r_highlight_rules.js for the list of keywords used in RStudio. It also comments out `package_other` line, as this list serves no useful purpose. 

Furthermore, the commit adds to `wordchars` the `.` (dot). In R the set of symbols which can be used in object names is `[A-Za-z0-9]` and `_` and `.` (with code portability and platform compatibility in mind). For more gruesome details see: http://cran.r-project.org/doc/manuals/r-release/R-intro.html#R-commands_003b-case-sensitivity-etc . RStudio also treats the dot as part of the "word". 

For some background see: http://landroni.wordpress.com/2013/07/27/using-geany-for-programming-in-r/
Please make a decision, don't simply comment it.  Also, we prefer to always set keyword lists, even if it's simply to an empty list.

So, what I'm asking is the removal of the comment, and either keeping, updating or removal of the actual value.  Removing it may however not please everyone, since there's probably a reason why it was added.