Add option to stop build removing dist directory

[eddyerburgh]

What problem does this feature solve?

I want to make two bundles in the dist directory using different config options. I wanted to do this by using an env var in the vue.config.js overrides, but the service scripts removes the dist directory each build.

What does the proposed API look like?

Add an option to stop the dist directory being removed:

yarn build --no-clean