Allow dev mode to use server rendering #301

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Feature request

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What problem does this feature solve?

Currently, saber runs your app in SPA mode, i.e. no server rendering, but the app produced by saber build will run in SSR mode (initial HTML is generated at build time).

The problem is sometimes some errors only occur in your production build, so it's hard to find or debug them during development.

What does the proposed API look like?

Add --ssr flag to saber command which allows the app to run in SSR mode.