ICONS for DMRS

[goodmami]

This issue is for adding individual-constraints to DMRS, for information structure and potentially other uses.

I'm spinning this issue off from #174 considering that it's a much more significant change than anything else in that issue, and similar to #177 in that it describes a new feature.

Discussion on the developer's mailing list happened here: http://lists.delph-in.net/archives/developers/2018/002786.html

The main conclusions are that ICONS do not always overlap with regular or scope-only links, or if they do may be multiply-linked or link in the reverse direction, therefore they should not be merged the way the scopal and non-scopal arguments are. Thus we will have separate "links" for icons, and until something like #177 is implemented, we cannot capture all possible icons.