Allow users to create a completely separate ES installation.

[quyykk]

Currently, every ES instance you download shares its save files and plugins with all the other instances. It would be very useful to have an option to specify that an instance should be self contained, i.e. it doesn't share any save files or plugins with the other instances.

[tehhowch]

Related #196

[xobes]

Just a thought, but the nature of the ES plugins would need to address this, unless we're expecting ES Launcher to do some sort of chroot or the like. What I mean is: the plugins are installed under a user's local files -- statically.