Add pre-commit config so formatR can be used as a remote hook

.pre-commit-hooks.yaml

@@ -0,0 +1,6 @@
+- id: formatr
+  name: formatR
+  description: Uses formatR to format R files
+  entry: scripts/pre-commit-formatR.R
+  language: script
+  files: '.*\.[rR]$'

README.md

@@ -14,3 +14,22 @@ to
 
 [![R source code after
 reformatting](https://db.yihui.org/imgur/TBZm0B8.png)](https://yihui.shinyapps.io/formatR/)
+
+## Pre-commit Hook
+
+formatR is available as a pre-commit hook. Note that, to use this, `formatR` and `docopt` must be installed in the system R library; the hook does not install anything itself.
+
+The following arguments are available:
+
+```
+Options:    
+  --no-comments         Remove comments. 
+  --no-blanks           Remove blank lines. 
+  --arrow               If given, arrows will not be substituted for = signs. 
+  --pipe                If given, pipes will not be changed. 
+  --brace-newline       If given, braces will not be put on new lines. 
+  --indent=INDENT       Number of indents. [default: 4]    
+  --no-wrap             Skip wrapping the text. 
+  --width-cutoff=WIDTH  Text width cutoff. [default: 80]    
+  --args-newline        If TRUE, arguments are started on a new line after a function call. [default: FALSE]    
+```

scripts/pre-commit-formatR.R

@@ -0,0 +1,31 @@
+#!/usr/bin/env Rscript
+
+library(docopt)
+library(formatR)
+
+doc <- "
+Usage:
+  pre-commit-formatR.R [options] <files>...
+
+Options:
+  --no-comments         Remove comments. [default: FALSE]
+  --no-blanks           Remove blank lines. [default: FALSE]
+  --arrow               If FALSE, arrows will not be substituted for = signs. [default: FALSE]
+  --pipe                If FALSE, pipes will not be changed. [default: FALSE]
+  --brace-newline       If FALSE, braces will not be put on new lines. [default: FALSE]
+  --indent=INDENT       Number of indents. [default: 4]
+  --no-wrap             Skip wrapping the text. [default: TRUE]
+  --width-cutoff=WIDTH  Text width cutoff. [default: 80]
+  --args-newline        If TRUE, arguments are started on a new line after a function call. [default: FALSE]
+"
+
+args <- docopt(doc)
+
+for (source_file in args[["files"]]) {
+    formatR::tidy_source(source = source_file, file = source_file, comment = !args[["--no-comments"]],
+        blank = !args[["--no-blanks"]], arrow = args[["--arrow"]], pipe = args[["--pipe"]],
+        brace.newline = args[["--brace-newline"]], indent = as.integer(args[["--indent"]]),
+        wrap = !args[["--no-wrap"]], width.cutoff = as.integer(args[["--width-cutoff"]]),
+        args.newline = args[["--args-newline"]])$text
+
+}