Add an extension system for pluggable build-time tasks

[samdze]

The idea is to explore whether/how NimScript can allow us to plug-in custom tasks to perform when the user builds the project.

Here's an examples of a use case, but can be applied to all kinds of assets pre-processing:

A task that searches for image files with a predetermined suffix and generates spritesheets containing rotations of the original images. Useful to optimize rotated images drawing on device.

The pluggable tasks could be other packages, but this is still to be decided depending on ease of use and amount of setup/configuration to be done.

An implementation of the use case above can initially be provided embedded in this module.

[ninovanhooff]

Somewhat related: #88