Add setting for initializing in a disabled state

[rparrett]

Fixes #10.

I was doing this in a local fork anyway to test an app, so I figured I might as well open a PR.

This does what is described in #10 (comment).

Side note: It's not totally clear to me why ParticleSpawnerData is not just included in ParticleSpawnerBundle. I experimented with removing it here: rparrett@d33aa0a and at a glance, things don't seem broken.

[mbrea-c]

LGTM, and examples run fine. Merging, thanks for the contribution!

Side note: It's not totally clear to me why ParticleSpawnerData is not just included in ParticleSpawnerBundle. I experimented with removing it here: rparrett@d33aa0a and at a glance, things don't seem broken.

If I remember correctly it used to be because I used scenes to build composite effects in an earlier build of my game, and before I made bevy_utilitarian I was using some types in ParticleSpawnerData that made it non-reflectable. Neither of these things apply anymore though, so it should be fine to put ParticleSpawnerData back in the bundle.