[feature request] allow custom preamble

[VincentTam]

TeXit has this feature. That would allow users to add whatever package/cmd they want, say PGF plots for automize axis setup, or a user-defined string for a specific color. A user sends a preamble modification request to the Discord bot, who would approve/reject that in a short period of time.

[tobiasBora]

Would be great to have. Even greater would be to allow people to use a whole file (attached?) as a preamble.