Not possible to specify a different game directory in the installer

[Pokechu22]

Some people have the Minecraft directory in a different location than %APPDATA%\.minecraft. However, the installer assumes this location (on windows; it uses a different hardcoded location on other OSes).

Being able to specify the location in the installer would be useful to these users; while this is a rarer setup it does exist and many other installers support it (e.g. the liteloader installer, the optifine installer, and the forge installer).

Related to #1.