brew audit: --strict warning for using $ in caveats

For multiple lines of commands to enter at a prompt, don't use
$ as a prompt marker to make it easier to copy/paste.

Library/Homebrew/cmd/audit.rb

@@ -233,12 +233,23 @@ def audit_formula_instance f
   return problems
 end
 
+def audit_formula_caveats f
+  problems = []
+
+  if f.caveats.to_s =~ /^\s*\$\s+/
+    problems << " * caveats should not use '$' prompts in multiline commands."
+  end if strict?
+
+  return problems
+end
+
 module Homebrew extend self
   def audit
     ff.each do |f|
       problems = []
       problems += audit_formula_instance f
       problems += audit_formula_urls f
+      problems += audit_formula_caveats f
 
       perms = File.stat(f.path).mode
       if perms.to_s(8) != "100644"