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