Add options to set the user/cache dirs

[RossBrunton]

This adds two new environment variables, FORGE_USER_DIR and FORGE_CACHE_DIR for overriding the default locations for user data.

For example, forge can be ran using:

FORGE_USER_DIR="$HOME/.local/state/forge" FORGE_CACHE_DIR="/tmp/forge" forge

To use alternate directories.

There should be no change in behaviour when neither of these vars are set.

[tool4ever]

hmn what's the point?
using the preferences file should already cover this a bit more easily

[RossBrunton]

I'm perhaps blind, but I can't see any documentation about a preferences file nor do I see a preferences file in the published build.

There's preferences information in the user dir, but that means that the user dir can't be relocated.

[tool4ever]

it's right above the point in the guide you've edited

[RossBrunton]

Had another look at this and it does look like that file can change the user data dir. For some reason I thought that file actually lived in the user dir and only controlled where the card images where stored. Sorry about that.

However, the properties file does have a few limitations:

• It has to be hardcoded in the forge package, so individual users can't change it unless they download their own copy of forge. If installed via a package manager, the installed package needs to be modified which is messy.
• As far as I can tell, it doesn't support the expansion of $USER or ~ (Or, presumably, %APPDATA% on Windows), meaning that the username has to be hardcoded.

With these two issues, I still think it makes sense to allow the user to override the locations using environment variables.