Add vendor chunk

[richardscarrott]

It'd be good to take advantage of LTC for node_modules as they're less likely to change so frequently.

The new code splitting PR adds a bootstrap chunk which could double up as a vendor chunk too.

Main issues are pulling out the chunk manifest from the webpack boostrap to ensure changes to other chunks don't invalidate the entry chunk. Also need to review records / deterministic re-builds (i.e. use hashes or names instead of indexes as module ids).

At the moment there's an issue when using the chunk-manifest-webpack-plugin as the chunkhash is generated before the plugin runs...workarounds could include the webpackmd5hash plugin...