Add ability to figure all JVM parameters

[reines]

Currently we hardcode a few, and only allow -Xms=-Xmx to be set. We should make this more flexible.

[siddharth96]

@reines Any update on this?

[reines]

Apologies, I'm not actively using Dropwizard anymore myself so development of this has fallen by the wayside. Happy to merge any PRs, but unlikely to get around to doing any changes on this myself.