Allow upgrades

[pjf]

At some point, people are going to want to upgrade their mods. There are two ways this is commonly done:

• Delete everything, and unzip afresh.
• Unzip in place.

The first means one knows exactly what one should end up with, the second means we can more easily preserve state generated by a previous install (although #15 provides a potential way of doing this).

We need to decide what the default behaviour is (right now it's unzip-in-place), but give the option to delete everything and unzip afresh.

[pjf]

Right now, upgrades don't work. You can't do them.

What we will be doing is:

• Uninstall the previous version (except preserved files, see Preserved files #15)
• Install the new one

This means mods have to be clear up-front about which files need to be kept between releases, and makes the upgrade process much more predictable.

I don't think we should allow unzip in place.

[AlexanderDzhoganov]

I have implemented this (except the preserved files part) in #83