Add flag to preserve build/compilation directories

[Stratus3D]

When a build fails it can be necessarily to look at the build directory after it failed. Debugging often requires inspecting logs and other files that may have been generated during the build.

Related to: #74

[Preen]

+1

[jthegedus]

Is this resolved with the always_keep_download setting? https://asdf-vm.com/#/core-configuration?id=homeasdfrc

[Stratus3D]

Good question. I suppose it technically isn't, but it should allow the user to repeat the commands run by the plugin. We should probably implement this flag as well, but it would be up to plugins to observe it and not delete the build directory.