add input for custom build command

[WickyNilliams]

I have a repo where the docs are colocated with source, but crucially not as a monorepo, just sibling directories. My npm run build command builds source, not astro. Of course i can && npm run build:astro or something at the end, but that just complicates things. Nor do I want to make astro my main build script. Would be nice if it were configurable somehow, but default to build for backward compatibility.

Happy to make a PR if you think this is OK

[WickyNilliams]

somewhat related to #29. since my astro site is in a /docs subdir, i don't think running the astro CLI directly would work for me. the astro CLI would not find my docs?

but maybe in that case the astro build could also be a configurable build command? so if you still want to use npm scripts you could override it?