allow plugins from parent project
allow plugins from parent project

So that projects that use lovr as a submodule can
inject their own plugins.

By picking them up from the _root_ project, whatever project that
is embedding lovr can decide for itself what plugins to use. This
is cleaner than using a separate glob and a variable in the case
where lovr will never come bundled with a standard set of plugins.

bjornbytes/lovr,bjornbytes/lovr,bjornbytes/lovr,bjornbytes/lovr,bjornbytes/lovr