Language::Node.npm_install_args: add helper method (#37)

* add Language::Node helper module

This adds a language module for Node module based formulas.
It contains the 2 public methods `std_npm_install_args(libexec)` and
`local_npm_install_args`:

* `std_npm_install_args` is intended to be used in formulas for
  standard node modules and returns `npm install` args for a global
  style module installation to libexec.
* `local_npm_install_args` is for formulas, in which the `npm install`
  step is only one of multiple parts of the installation process and
  returns `npm install` args for a default local installation in place.

Both methods have in common, that they are

* making sure that a working copy of npm and node-gyp from node's
  libexec is prepended to the PATH (to not rely of a user managed npm)
* seting the npm cache to HOMEBREW_CACHE/npm, which fixes issues caused
  by overriding $HOME resulting in long install times + high disk usage
  (see https://github.com/Homebrew/brew/pull/37#issuecomment-208840366)

* audit: update npm install check for Language::Node

* cleanup: remove npm_cache too

* doc: add Node-for-Formula-Authors.md

1 files changed, 4 insertions, 7 deletions

diff --git a/Library/Homebrew/cmd/audit.rb b/Library/Homebrew/cmd/audit.rb
index eca4fbb1e..64b61ec22 100644
--- a/Library/Homebrew/cmd/audit.rb
+++ b/Library/Homebrew/cmd/audit.rb
@@ -656,13 +656,6 @@ class FormulaAuditor
     if text =~ /def plist/ && text !~ /plist_options/
       problem "Please set plist_options when using a formula-defined plist."
     end
-
-    if text =~ /system "npm", "install"/ && text !~ %r[opt_libexec\}/npm/bin] && formula.name !~ /^kibana(\d{2})?$/
-      need_npm = "\#{Formula[\"node\"].opt_libexec\}/npm/bin"
-      problem <<-EOS.undent
-       Please add ENV.prepend_path \"PATH\", \"#{need_npm}"\ to def install
-      EOS
-    end
   end
 
   def audit_line(line, lineno)
@@ -885,6 +878,10 @@ class FormulaAuditor
       problem "Use `assert_match` instead of `assert ...include?`"
     end
 
+    if line =~ /system "npm", "install"/ && line !~ /Language::Node/
+      problem "Use Language::Node for npm install args"
+    end
+
     if @strict
       if line =~ /system (["'][^"' ]*(?:\s[^"' ]*)+["'])/
         bad_system = $1