Add module API

1. Can now be consumed as a standard es module too:

import { build, watch } from 'https://deno.land/x/[email protected]/src/build.ts';

2. CLI flags have changed:

Before	After
--dev	--watch (to match export name)
--site	--site_dir
--build	--out_dir
	--minify toggles minification (defaults to true, but false in watch)

3. The default build output is now /build.

Full Changelog: 0.5.0...0.6.0