Add biFlatMap to EitherT

[ochrons]

EitherT has a bimap but no biFlatMap. This would be useful when chaining computations and needing to process both sides of the Either with functions that return an EitherT. For example:

def findVertex(id: String): EitherT[Future, APIError, Vertex]
def addVertex(...): EitherT[Future, APIError, String]
def doSomethingWithVertex(...): EitherT[Future, APIError, String]

findVertex("id").biFlatMap( {
  case _: APIError.NotFound => addNewVertex(...)
  case error => EitherT.left(error)
},
vertex => doSomethingWithVertex(vertex)
)

[agajek]

I can take this one.