add support for programmatic usage and client only builds (i.e. for electron target)

[alidcast]

ref #392 and #320 (issues requesting electron support, shut down because razzle runs two bundles by default)

seems like there are two issues with using razzle with electron

1. there's no programmatic usage
2. there's no option to only run client bundle

would you be interested in a PR adding these two options? it would align with other SSR built tools like next.js and nuxt.js which also offer the above and thus support electron as a target

I currently have a fork that has a working version of this. it extracts some of the logic inside scripts so that the client bundle can be compiled programmatically, and has a minor change in createConfig to fix an error that was occurring in electron window.