Use the user directory as default config file location

[eternal-flame-AD]

The current default(first) location for configuration storage is ./cli.json, which is not so useful in most cases since the user would need to:

• Call gotify-cli in this specific directory for every invocation
• Create a cli.json in a random directory, which is not a clearly explanatory file name.

So I think in most cases, the user would use user directory as their config storage, so we should put that in front of ./cli.json by default during initialization.