Added :sass_cache_path custom config of SASS cache directory

The default output of SASS .sass_cache directory is in the Middleman app root directory, which to my mind adds clutter to the directory.

Secondly, when storing Middleman apps in a Dropbox subfolder, the .sass_cache directory gets synced unnecessarily.

This fix enables moving the .sass_cache directory to any path on the system, such as the "/tmp" directory for automatic discarding of files when restarting the system.

Suggested usage:

    #in config.rb
    set :sass_cache_path, File.join('/tmp', "middleman-#{File.basename(Dir.pwd)}", "sass_cache")

which could return something like:

    `/tmp/middleman-example.com/sass_cache`

This keeps multiple projects separated and easily identifiable.

middleman-core/lib/middleman-core/application.rb

@@ -101,6 +101,11 @@ module Middleman
     # @return [String]
     set :css_dir,     "stylesheets"
 
+    # Location of SASS .sass_cache directory.
+    # @return [String]
+    #   set :sass_cache_path, "/tmp/middleman-app-name/sass_cache"
+    set(:sass_cache_path) { File.join(root_path, '.sass_cache') } # runtime compile of path
+
     # Location of images within source. Used by HTML helpers and Compass.
     # @return [String]
     set :images_dir,  "images"

middleman-more/lib/middleman-more/core_extensions/compass.rb

@@ -21,7 +21,7 @@ module Middleman
             ::Compass.configuration do |config|
               config.project_path    = source_dir
               config.environment     = :development
-              config.cache_path      = File.join(root, ".sass-cache")
+              config.cache_path      = sass_cache_path
               config.sass_dir        = css_dir
               config.css_dir         = css_dir
               config.javascripts_dir = js_dir