Allow user configuration and support XDG spec by default

[marwanhawari]

This PR introduces the ability for users to configure the stewPath (which is where stew data is stored - originally hardcoded as ~/.stew) and the stewBinPath (which is where binaries are actually installed - originally harcoded as ~/.stew/bin). There are 3 ways to configure these:

1. Upon initial installation with the install.sh script, it will prompt you to configure these.
2. If you install the stew binary without the install.sh script or later realize you want to change them, you can use the new stew config subcommand.
3. You can manually edit the stew config file ($XDG_CONFIG_HOME/config.json or ~/.config/stew/config.json on Linux/macOS and ~/AppData/Local/stew/Config/config.json on Windows)

Additionally, the new default locations for stewPath and stewBinPath follow the XDG spec:

	Linux/macOS	Windows
stewPath	$XDG_DATA_HOME/stew or ~/.local/share/stew	~/AppData/Local/stew
stewBinPath	$XDG_DATA_HOME/stew/bin or ~/.local/share/stew/bin	~/AppData/Local/stew/bin

[marwanhawari]

Resolves #4