Allow for changing systemsettings file name

[KamilKurde]

This pull request...

• Fixes a bug
• Introduces a new feature
• Improves an existing feature
• Boosts code quality or performance

Description

This pull request adds a possibility to change serversettings filename

Purpose

Making it easier to host multiple bot instances, by eliminating the need for them to have their own working directories with serversettings.json that's not going to be modified by other instances

[jagrosh]

This was discussed briefly in Discord and this is a feature that we want added, but it should be set from the config file instead of a system property. Using the config won't prevent you from using a system property (as HOCON can read system properties and environment variables)