[Feature Request] Allow users to set a custom 'serversettings.json' path

**Is your feature request related to a problem? Please describe.**
At this moment, I believe there is no way to set a custom path for the `serversettings.json`
For the `config.txt` this is possible by adding `-Dconfig` to the command line, but for `serversettings.json` this is not possible.

**Describe the solution you'd like**
I would like to see a new command line argument called `-Dserversettings` which can point to a location where the `serversettings.json` should be saved, similar to `-Dconfig` which points to the `config.txt`

**Describe the basic flow/steps of using this feature**
How this feature would be used:
1. Users will obtain the ability to set a custom `serversettings.json` path using -Dserversettings

**Additional context**
none

**Please check the following links to confirm that your feature has not already been requested:**
https://github.com/jagrosh/MusicBot/labels/enhancement
https://github.com/jagrosh/MusicBot/wiki/Things-That-Won%27t-Be-Added

Start by locating how the existing -Dconfig argument selects config.txt and where serversettings.json is currently resolved or saved. Trace those entry points, then verify that -Dserversettings selects the requested path without changing the existing default behavior.

Written by the indexing model from the issue text.