Use the user directory as default config file location

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.