Add way to set global vars (*assert*, *warn-on-reflection*, etc.) through config key :global-vars.

All the tests still pass. I have kept the `:warn-on-reflection` option for backwards compatibility.

I have tested this on a small example project and global variables do indeed seem to propagate correctly. Global variables have to be set before `~init`, however, lest the `*assert*` flag is useless (it is used at compile-time, so it must be set *before* files are *required*).