Add `mod-flows` action

Hi,

I recently discovered this project in search for a way to manipulate flow tables on OVS using Golang.

The `AddFlow` function works well for me, and I suppose that instead of modifying a flow, I could delete it and add the new one.

Yet, I wanted to ask if there is a reason for not supporting `client.OpenFlow.ModFlow()`? Or does it do delete and add internally anyway? Maybe I could contribute this functionality if there is a reason to do so.

Hope to hear from you soon!
LaumiH