Add feature to disable status updates

[kbarber]

Some of us use this plugin combined with Travis CI which dominates the status field also. In effect the ghprb plugin doesn't support switching off attempting to use the status field unless you modify permissions (causing a 'silent' failure) whereby it falls back to commit messages.

I propose that a useful use-case is that the commit message is the only mechanism, so it would be nice to have an option to disable the status update without doing something like say: removing the push permissions from the gh user.

[bilderbuchi]

FWIW, and slightly tangential to this, Github will get combined statuses soon, so that multiple statuses can coexist. Also, there's a repo:status OAuth scope nowadays, so push permissions should not be necessary anymore to set commit statuses.

[bchavet]

This would be an amazing feature! This is basically the one thing that is making us consider using something other than Jenkins/GHPRB. Is there anything I could do to help push this along?

[lifeofguenter]

Would love to be able to disable default statuses as well!

[jcfj]

Is this related to #407 ?