add CacheFile backend module

[wjordan]

This PR adds a new backend module CacheFile. This module overwrites the Base#load_file method to store metadata of loaded file contents, and use this metadata to skip re-loading the same files when their contents haven't changed.

• The module works with Simple and KeyValue backends, though it's probably more useful in the latter (where the translation store persists across application restarts).
• Loaded-file metadata is stored in the i18n backend itself, using the :18n locale :load_file scope, with filename as key.
• When determining whether a file's contents have been changed, it first checks the file-modification time (using File.mtime(filename).to_i), and if they don't match, it additionally checks the file's content (using Digest::SHA2.file(filename).hexdigest). The second check avoids re-loading 'touched' files when the contents haven't changed.
• I haven't committed to the name yet, could use other suggestions.
• The tests provide 100% line-by-line coverage, across both Simple and KeyValue backends.
• My use-case for developing this backend module is to improve per-process memory-usage (and less important, startup time) of a large, heavily-localized Rails application loading ~500mb of localization data stored in .yml files, with minimal application changes.

[radar]

Seems pretty sensible to me. Thank you @wjordan!