core: put shell quoting into its own util class, it is useful

lib/vagrant/util/shell_quote.rb

@@ -0,0 +1,15 @@
+module Vagrant
+  module Util
+    module ShellQuote
+      # This will auto-escape the text with the given quote mark type.
+      #
+      # @param [String] text Text to escape
+      # @param [String] quote The quote character, such as "
+      def self.escape(text, quote)
+        text.gsub(/#{quote}/) do |m|
+          "#{m}\\#{m}#{m}"
+        end
+      end
+    end
+  end
+end

test/unit/vagrant/util/shell_quote_test.rb

@@ -0,0 +1,12 @@
+require File.expand_path("../../../base", __FILE__)
+
+require "vagrant/util/shell_quote"
+
+describe Vagrant::Util::ShellQuote do
+  subject { described_class }
+
+  it "quotes properly" do
+    expected = "foo '\\''bar'\\''"
+    expect(subject.escape("foo 'bar'", "'")).to eql(expected)
+  end
+end