R: Fix keywords and wordchars

R: Fix keywords and wordchars

Currently the R filetype has a rather random collection of keywords.

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.

Furthermore, the commit adds the `.` (dot) to `wordchars`.  In R the
set of symbols which can be used in object names is `[A-Za-z0-9_.]`
(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/

Closes #273.