Add the ability to restrict matching to possible linear followups

[jvoisin]

It would be great to be able to tell weggli that all the predicates have to be following each other in a linear way.

For example:

if (a)
  A;
else
  B;
C
if (b)
  D;
else
  E;

Here, A can't follow B, but it can be followed by C and D or E.

This should make queries like free($a); free($a); more interesting.