refactor(gulpfile): flatten if-else in make()
feat(gulpfile): generate chromium manifest

Rewrite build/chromium-manifest.js to ./manifest.litcoffee

Remove "permissions" field as it works correctly without
explicit permissions.

Remove "css" field from content script definition as we bundle styles
within dist.js

Add manifest() to gulpfile, call it in make() so manifest is generated
in both "default" and "make" tasks.
fix(gulpfile): rename manifest to generate in manifest()

CoffeeScript, why so implicit?
fix(gulpfile): in manifest() check that build/chromium exists