Support multiple clusters

[cjimti]

In response to #24 I am opening up this issue to support multiple cluster forwarding.

[cjimti]

@mrumpf I have a work in progress version of multiple cluster support in the branch 1.5.x.

Examples

Where cluster1 and cluster2 are kubectl contexts:

Forward mynamespace from cluster1 (as .cluster.local) and cluster2 as .cluster.cluster2):
sudo kubefwd svc -n mynamespace -x cluster1 -x cluster2

Forward somenamespace and mynamespace from cluster1 (as .cluster.local) and cluster2 as .cluster.cluster2):
sudo kubefwd svc -n mynamespace -n somenamespace -x cluster1 -x cluster2

Omitting the -n will use the default namespace from the first context.

Since this use case diverges a bit from the original rationale for kubefwd, I hope to keep it backward compatible. I see how this feature could be useful for testing but I think it would need quite a bit of work to find service names for federated clusters.

If anyone has an opinion on the host entry names for the additional clusters, let me know. For now, I use the context name since it will be unique.

[cjimti]

Multi-cluster support is now in 1.7.x