[Linux] change the save directory of Hive.initFlutter()

**Is your feature request related to a problem? Please describe.**
The Hive.initFlutter() method sets the directory for the database files to '$HOME/Documents'. On Linux systems user-specific application files should be saved to the '$HOME/.local/share/' directory. This directory can be retrieved by the path_provider package with the getApplicationSupportDirectory() method.

**Describe the solution you'd like**
On Linux systems the Hive.initFlutter() method should call the getApplicationSupportDirectory() to retrieve the '$HOME/.local/share/' directory.

If the change is appreciated i can create a Pull request.

**Version**
- Platform: Linux
- Flutter version: [e.g. 1.5.4]
- Hive version: [e.g. 0.5.0]