Add support for induced bipartite subgraph

As part of a project, I had to use bipartite induced subgraphs of a graph.

Let $G=(V,E) $ be a graph and let $X,Y\subseteq V$ such that $X \cap Y = \emptyset$. The bipartite subgraph of G induced by $X$ and $Y$ is $$(X \cup Y, X \times Y \cap E) $$
In other words, it's the bipartite subgraph made of all the edges of $G$ that are between $X$ and $Y$.
Such a construction is useful in, for example, the search of vertex separators.

I've made a simple implementation (30 lines or so) for it and I think it might be worth including it.
Before making a pull request, I wanted to see if this is something people might find interesting to have in the package.

The issue names no files, tests, or entry points, and the proposed implementation is not included. Begin by locating Graphs.jl's graph-construction API and test suite, then verify the induced edge set for disjoint X and Y; done means the feature is exposed and its behavior is covered by tests.