Add support for VTA algorithm

[OscarBohlin]

The VTA algorithm is yet another way of constructing a call graph which in some cases produces more precise call graphs than RTA/CHA.

The changes needed are:

• Add the required package as dependency
• Add parameters and necessary struct entries
• Add switch option for the vta algorithm. This needs to fall through to rta because vta needs an initial graph and a map[*ssa.Function]bool of which functions to search for. We pass the result from rta to vta. The vta call also includes option for an already constructed (initial) graph, which we pass the already constructed one by rta.
• Update documentation in README.md to include new algorithm