upgrade build process

[phivk]

The outdated version of Grunt is causing a lot of security issues.
We could look into updating Grunt's major version, but I feel like a grander rewrite of the build process might be more future proof.

See for inspiration the node-based build process used in Snowman:
https://github.com/videlais/snowman/blob/master/scripts/build-format.js

Not a lot of experience here, so any help is appreciated.