Support external custom Formatters

[brittf19]

🤔 What's the problem you're trying to solve?

I have multiple GoDog testing projects and a custom formatter that i wrote for them. But, i unfortunately found out that a custom formatter must be part of the testing package and compiled with the test files as part of the package, otherwise GoDog fails with an error: "unregistered formatter". I would like to suggest an option for loading a precompiled formatter that is independent of the testing package.
I would like to use this formatter in various different testing projects. This puts me in a situation that i need to copy the source files to each project and recompile the tests. Its harder for me to change my code and follow up the formatter's bugs.

✨ What's your proposed solution?

Add an option to load a formatter package. something like:
-f path/to/formatter.bin would be nice. even maybe add a flag for an outside formatter if it would be easier for you to add an option and not adjust the existing one.

⛏ Have you considered any alternatives or workarounds?

I'm currently copying the files to each testing project package/directory and recompiling.

[vearutop]

Is it correct that you're using godog in CLI mode? Please have a look at #478 and share your vote/opinion.

This feature does not seem that much feasible since all solutions to create precompiled plugins in Go impose limitations.

My suggestion is to add your custom formatter to a library module (or create a new one) and use it as a dependency in your projects, for example like it is done here.