Add the ability to register commands to be run on start up or passage change

[Philip-Sutton]

Add the ability to register commands to be run on start up or passage change. This would allow for automation of tweego or other compilers / post processors.

I use a batch file to run a terminal command to run tweego in watch mode. If I forget to run this batch file, then (of course) tweego doesn't do the building. It would be great to be able to automate this within T3LT.

[cyrusfirheir]

VSCode has tasks. Along with the ability to run them on startup.

[Tranberry]

@MinusGix didn't you know that? 👁️ 👺