build.js: support flags

Adds flags to build.js to use rsync, disable linting and use watch mode.

(cherry picked from commit 3b14e61390)

README.md

@@ -57,7 +57,7 @@ You can also use
 [watch mode](https://esbuild.github.io/api/#watch) to
 automatically update the bundle on every code change with
 
-    $ npm run watch
+    $ ./build.js -w
 
 or
 
@@ -118,7 +118,7 @@ During fast iterative development, you can also choose to not run eslint/styleli
 This speeds up the build and avoids build failures due to e. g. ill-formatted
 css or other issues:
 
-    $ make LINT=0
+    $ ./build.js -es
 
 # Running tests locally