Add knitr and R markdown patterns to R.gitignore

These patterns handle cache diretories and temporary files created
by the widely used [knitr](http://yihui.name/knitr/) and [R markdown](http://rmarkdown.rstudio.com/)
packages.

R.gitignore

@@ -23,3 +23,11 @@ vignettes/*.pdf
 
 # OAuth2 token, see https://github.com/hadley/httr/releases/tag/v0.3
 .httr-oauth
+
+# knitr and R markdown default cache directories
+/*_cache/
+/cache/
+
+# Temporary files created by R markdown
+*.utf8.md
+*.knit.md