use the systems data directory as default_path

Most operating systems have a **default path**, where programs can store their data, e.g. in Linux this is `~/.local/share/` for data and `~/.config/` for configuration files. It would be nice if cheat supports this, instead of using the `.cheat` directory.

I think we should look at three cases:

1. If the environment variable is set, use it:

   If the user sets it, he or she probably done it for a reasons, so use it in any case.

2. If a `.cheat` directory exists in the home directory use it:

   Of course for backward compatible reasons and as an easy way for people who like this 
   method better than 3.

3. If none of the above, use the systems default dir:

   This is imo the "cleanest" solution and should be the default.