Support path stripping on install.

[pjf]

Often we end up with install stanzas which look like this:

{
    "file"       : "MyMod-1.1.1/MyMod",
    "install_to" : "GameData"
}

This currently makes GameData/MyMod-1.1.1/MyMod, when we want it to create GameData/MyMod.

Paths which already contain GameData in them are handled by our current code, but it would be good to provide an explicit strip section which indicates what should be stripped off when installing:

{
    "file"       : "MyMod-1.1.1/MyMod",
    "install_to" : "GameData",
    "strip"      : "MyMod-1.1.1"
}

This would also assist in #259.

[pjf]

As invultri pointed out on IRC, is there any issue if we just strip out everything up until the last segment of the path?

[pjf]

As of PR #287, we now do path stripping automatically. So unless I'm mistaken, this is done. :)

(Checking with @christianvdstap just in case.. please re-open if I'm wrong here!)