Change the way default data directory is calculated

Based on the discussion in https://github.com/chrisallenlane/cheat/issues/142 I refactored the `default_path` function in `sheets` and added two functions to `utils`:

* `get_userdata_dir`
    
  returns the default user data directory: from envvar, in homedir or in standard dir.

* `get_default_data_dir`

  returns the default directory used for user data: chooses based on system and user.

Now which directory is used to store the user sheets, is determined the following way:

1. If the `DEFAULT_CHEAT_DIR` environment variable is set, use it.

2. If a `.cheat` directory exists in the home directory, use it.

3. Use a `cheat` directory in the systems default directory for user data.
might seem overkill to create a extra function for this, but maybe you have to use another function / library or method on mac or windows. we could add plattfor code here...