issue: fix nil pointer dereference on plumb error
plumbserve() fails with a nil pointer dereference when it tries to print errors via w.Err(). This happens because w is a dummy window with a nil *acme.Win. This commit replaces w.Err() calls in plumbserve() with acme.Errf(). It also extracts the plumbserve() method out of *awin, given that the previous change removes this dependency.
This PR also documents "githubissue" plumb port requirement and how to create it under the section "Acme Editor Integration".
Fixes issue #5.
The initial version of the PR was using log.Printf() to output plumb errors. I've updated it to use acme.Errf() instead.
Got halfway implementing this same patch then found this one. Thanks @jroimartin :)