Add BrowserFetcher bindings

[jihchi]

Fix #40

--

These bindings increase our unit test duration.

[zploskey]

These look good to me. It's debatable whether we should bother typing the platform return value beyond a string. I doubt this will be used often. Would be fine with it as is.

[zploskey]

win64 should be all lower case. If you just make the polymorphic variants all lower case and the same as the string doesn't it convert them to the right string? No need to use bs.as.

[jihchi]

I thought variant constructor need to be capitalized. tried lower case but without success.

[zploskey]

Polymorphic variants don't need to be capitalized. For example: https://github.com/bs-puppeteer/bs-puppeteer/blob/master/src/ConsoleMessage.re

[jihchi]

Oh! That works, Thanks!

[jihchi]

Hi @zploskey ,

PR is done.

[zploskey]

To be consistent this should be makeBrowserFetcherOptions, plural.

[jihchi]

Fixed in commit 784ba5c