Use package-wide options to set some preferences

Those are things that users might what to change often:

- the path where devel packages are installed: `~/R-dev` is a 
  little intrusive

- the github user name: developer might want to use theirs

R/devel-mode.r

@@ -19,7 +19,7 @@
 dev_mode <- local({
   .prompt <- NULL
 
-  function(on = NULL, path = "~/R-dev") {
+  function(on = NULL, path = getOption("dev.path")) {
     lib_paths <- .libPaths()
 
     path <- normalizePath(path, winslash = "/", mustWork = FALSE)

R/install-github.r

@@ -14,7 +14,7 @@
 #' \dontrun{
 #' install_github("roxygen")
 #' }
-install_github <- function(repo, username = "hadley", branch = "master", subdir = NULL, ...) {
+install_github <- function(repo, username = getOption("github.user"), branch = "master", subdir = NULL, ...) {
   message("Installing github repo(s) ", 
     paste(repo, branch, sep = "/", collapse = ", "), 
     " from ", 

R/zzz.r

@@ -64,3 +64,18 @@ current_ver <- function() {
   paste(version$major, ".", minor[1], sep = "")
 }
 
+.onLoad <- function(libname, pkgname)
+{
+  op <- options()
+  op.devtools <- list(
+    dev.path="~/R-dev",
+    github.user="hadley"
+  )
+  toset <- !(names(op.devtools) %in% names(op))
+  if(any(toset)) options(op.devtools[toset])
+}
+
+on_path <- function(...) {
+  unname(Sys.which(c(...)) != "")
+}
+