Add functions for accessing/creating files

[andybarron]

• get_app_file would be identical to get_app_dir.
• app_file would:
  • Call create_dir_all on the path's parent.
  • Call OpenOptions::new().create(true).append(true).open on the full path.
  • Return the full path.
• Could definitely consider combining get_app_dir and get_app_file into get_app_path and deprecating the old names. But I think that confuses the API. Even if the difference between get_app_dir and get_app_file is purely semantic, it's good (IMO) to mirror the difference between app_dir and app_file.

[calidion]

Is it implemented?