Rcpp::compileAttributes

Hi,

since Attributes are becoming a de-facto standard for Rcpp based packages

http://cran.r-project.org/web/packages/Rcpp/vignettes/Rcpp-attributes.pdf
http://adv-r.had.co.nz/Rcpp.html#rcpp-package

a useful add to `maker` (IMHO) could be `Rcpp::compileAttributes()` (that does the Rcpp's before-compilation-voodoo to make `C++` files in `src/` automagically available in the forthcoming R package).

Cheers,
 Luca