Add Tarjan's algorithm

[ZigRazor]

Tarjan's algorithm for finding strongly connected components in a directed graph
Uses two main attributes of each node to track reachability, the index of that node
within a component(index), and the lowest index reachable from that node(lowlink).
We then perform a dfs of the each component making sure to update these parameters
for each node and saving the nodes we visit on the way.
If ever we find that the lowest reachable node from a current node is equal to the
index of the current node then it must be the root of a strongly connected
component and so we save it and it's equireachable vertices as a strongly
connected component.

[gaurang7goel]

HI, i can do this. Can you assign it to me.

[gaurang7goel]

@ZigRazor Please assign it someone else, as I was not able to understand the issue properly .

[ZigRazor]

ok @gaurang7goel