Alternative for "browser" key with higher priority

I've stumbled upon this issue in https://github.com/mwilliamson/mammoth.js/pull/106

`browser` key is widely used for defining which file should be used in browser environment. Some tools might use this field optionally, but they also have an alternative which is tool-specific, for instance, [in webpack](https://webpack.github.io/docs/configuration.html#resolve-packagemains).

Could you add support for something like `browserify.browser` with higher priority in addition to `browser`?