Add a security manager to the Sandbox class to prevent file access

[irufus]

For example, you should be able to prevent calls like
!jshell java.util.Arrays.toString(new java.io.File(".").listFiles());
OR
!jshell java.util.Arrays.toString(new java.io.File("/home/").listFiles());
from listing files on the system that it is running on.

We would need to add this option to the example configuration file. bot.properties.example.