Support rpc call cancellation/control via context.Context

Currently actions executed in the plugin cannot be cancelled as the RPC client `Call` function does not support the use of context.
```go
g.client.Call("Plugin.Greet", new(interface{}), &resp)
```
This could be problematic if the action takes a long time and we would like to control it via deadline/timeout
The only possible option seems to be kill the whole plugin server unless I missed something.

Would it be possible to leverage packages such as https://github.com/keegancsmith/rpc which propose similar clients to rpc.client but which support `context.Context`. This way we could use the context to cancel calls or to add a timeout protection