Add exports so build can be consumed as a module

[sndrs]

added the ability to consume build as a module:

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

In the process, some 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)

The default build output is now /build – I kept finding it hard to navigate to a folder that appeared and disappeared in src. happy to revert this if it's not popular.

[mxdvl]

Yes, very nice!

The default build output is now /build

I did a similar thing in another branch (maybe #29 ?)