Add `mod-flows` action

[LaumiH]

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