Write a function that determines if a path exists between two vertices of a graph.
The graph will be represented as an object, each of whose keys represents a vertex(node) of the graph and whose value represents all vertices that can be reached from the aforementioned key. In the example below, there is a connection from vertex a to vertex b and a connection from vertex b to vertices c and d but not a connection from vertex b to vertex a.
{a: ['b'],
b: ['c', 'd'],
c: ['d']