Highlight modules with signatures

[Lysxia]

Module A : B.

The part after the colon is currently not recognized. The regexp for modules looks a bit too tricky to understand at the moment.

[Tuplanolla]

This problem also plagues module subtype relations, as seen in Coq's own theories/Arith/PeanoNat.v.

Module Nat
 <: NAxiomsSig
 <: UsualDecidableTypeFull
 <: OrderedTypeFull
 <: TotalOrder.