Allow users to add their own keys to message-colors

[ghost]

Feature description

Allow users to add their own keys to message-colors f.e.:

message-colors: `

primary: '#ffaa00'
secondary: '#ff5555'
infoWithPrefix: '<dark_gray>[<#ffae3a>MyServerPrefix<dark_gray>] <gray>'

` make infoWithPrefix work in messages like <infoWithPrefix> or something

How the feature is useful

That would make messages file more simple and less confusing for server owners