Add "browser" environment variable to handle incompatibilities

[emilgoldsmith]

As talked about in #79 and #52.

[martynchamberlin]

Not browser. OS!

[martynchamberlin]

Oh. Sorry. Got mixed up what we were talking about. Yes, for this, browser. :)

[emilgoldsmith]

Haha yeah. I guess we can only detect OS by javascript, not by distribution ;).

We can just add the environment variable in the webpack builds as we at build time know what the target browser is.