Plugin support

[rxt1077]

Addresses #974.

This is a rudimentary version of plugin support for libasciidoc. It adds a command-line option to the binary and runs a PreRender function from plugins just before the document is rendered. Plugins are golang plugins with the PreRender symbol exported.

[xcoulon]

thanks for this PR @rxt1077
I need to take a closer look at Go plugins, since they seem to have quite a few limitations :/
Also, could you include some tests to show and verify how it works? This may require some additional goals in the makefile, though 🤔

[rxt1077]

Thanks and I understand. Go plugins do seem limited and it's worth exploring other options. This is less of a "best" solution and more of a "possible" solution. I appreciate you taking a look and please don't feel pressured towards any action. I'll see if I can create some tests, I realize that part of my PR is missing.

[rxt1077]

I wrote an example plugin, changed the make recipes to compile it, and added tests for loading a plugin. Despite that, it seems that I can't load plugins from the ginkgo tests without having the exact same runtime as the rest of the library. I'm not entirely sure when/how the ginkgo tools are compiled.

At this point I'm almost 100% sure that golang native plugins aren't the best solution for this. They seem to make it too difficult for other contributors to write plugins that can be loaded. They really seem to be meant to be exclusively used in-tree in very large projects.

Unless there are any objections, I'm going to close this PR and try again using go-plugin, which basically serves golang interfaces over local RPC. It seems like a much better way of doing it.

[xcoulon]

ok, thanks for the update and for all the time you spent on this, @rxt1077
I haven't tried go-plugin yet, but I will try to give it a look as well.