Bundle analyzer

[afoxman]

If we need detailed bundle analysis for Metro, we're going to have to piece together the tooling ourselves.

Webpack has a plugin for bundle analysis: https://github.com/webpack-contrib/webpack-bundle-analyzer. This looks nice and is familiar to the community. Replicating this in Metro would be ideal.

Alternatively, there is a library which does bundle analysis using the source-map file: https://github.com/danvk/source-map-explorer. This could be used as a starting point, or as an alternative "feed" to power the webpack analyzer.