New option for subcommand 'issue update' : '--add-label'

[RichardHitier]

In case of adding a label to already labelled issue, I find it unhandy to retype all previous labels I want to keep.
Maybe ' --add-label ' could simply do the trick

[leandro-lucarella-sociomantic]

I think we could use some special formatting, like issue update --label +to_add --label -to_remove.

We will have to use the API endpoint for labels instead of updating the issue for this, otherwise we have a race condition when setting new labels.

Here are the API calls we should use:

• https://developer.github.com/v3/issues/labels/#add-labels-to-an-issue
• https://developer.github.com/v3/issues/labels/#remove-a-label-from-an-issue