Always infer debug mode from flag

[marvinhagemeister]

I can't exactly reconstruct why I accidentally uploaded a debug build for chrome yesterday when cutting a new version (see #467 ). My assumption is that I ran the run:chrome command which builds a debug extension and loads it into a fresh chrome instance. I typically always use that just before publishing to make a final smoke test check. Normally, I run the build:chrome command once that's done but it looks like I forgot that yesterday. That's very likely what happened yesterday.

To prevent such accidents in the future this PR makes a couple of changes:

• Always use separate directories for debug and non-debug builds
• Always add -debug suffix to zipped debug extensions
• Use a global variable that must be injected to toggle debug mode instead of relying on process.env.DEBUG