embed_bqm should accept an edge list for it's target graph in addition to adj

**Current Problem**
A common format for graphs is an edgelist and we can support that with `embed_bqm`, `embed_ising` and `embed_qubo`.

**Proposed Solution**
It would be nice if we could provide an edgelist so
```
emb = embed_bqm(bqm, embedding, [(0, 1), (1, 2)])
```
would work