Update .gitignore

Using recommended defaults for R

.gitignore

-.Rproj.user
+# History files
 .Rhistory
+.Rapp.history
+
+# Session Data files
 .RData
+
+# User-specific files
 .Ruserdata
-src/*.o
-src/*.so
-src/*.dll
+
+# Example code in package build process
+*-Ex.R
+
+# Output files from R CMD build
+/*.tar.gz
+
+# Output files from R CMD check
+/*.Rcheck/
+
+# RStudio files
+.Rproj.user/
+
+# produced vignettes
+vignettes/*.html
+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