Only use wasm-opt when in release mode

[thedodd]

This addresses a few pain points where there was no way to reasonably
disable wasm-opt use for debug builds while keeping it enabled for
release builds. There seems to be very few (if any) use cases for using
wasm-opt on a debug build, as such, wasm-opt is now only run on release
builds when enabled.

closes #197
closes #175

Checklist

• Updated CHANGELOG.md describing pertinent changes.
• Updated README.md with pertinent info (may not always apply).
• Squash down commits to one or two logical commits which clearly describe the work you've done. If you don't, then Dodd will :).