Allow user to override location of configstore directory using an environment variable

[MatejKafka]

Some users may have different opinion on where configstore should store the data. For example, using C:\Users\<name>\.config on Windows is unconventional and not really common, nor recommended (actually, it's an anti-pattern that only Linux tools use).

No matter the reason, the user may want to store the config somewhere else (my usecase is for portable packaging of Node.js, where I want it to store everything in a custom directory). Imo, configstore should support an environment variable like CONFIGSTORE_DIR and store the config elsewhere if it is set.