Add browser support

[Richienb]

Fixes #60

cc @sindresorhus

[sindresorhus]

See https://nodejs.org/api/esm.html#esm_conditional_exports on how to add the browser version.

[Richienb]

@sindresorhus Should I be concerned that browserify doesn't detect the browser version this way?

[sindresorhus]

We can have both the browser field and the conditional exports. I would recommend opening an issue on Browserify about supporting conditional exports though. That's the future.

[Richienb]

@sindresorhus Done!

[sindresorhus]

Since the browser version is just for modern browsers, we can just use ES2015 export syntax there.

[Richienb]

Fixed!

[Richienb]

@sindresorhus Please re-review.