Feature request: Implement "CG NEXT"

[jcalado]

Documentation: https://github.com/CasparCG/help/wiki/AMCP-Protocol#cg-next

Upon receiving a "CG NEXT" command, a function defined on the template is called.
This way templates that receive large chunks of data can split them and go through the chunks each time "next" is issued.
Or an animation can stop somewhere, and continue upon being issued "next".