Use "Blocking" label

[per1234]

I'd like to suggest adding a new label Blocking which is to be added to any issue or pull request that must be closed before the next IDE release. There are so many issues/PRs that important ones can easily be buried and forgotten. I know you already use milestones but these don't seem to be strictly followed and won't work for the other IDE component repositories. An example of an issue that would have benefited from this system is #3795 which originally had the 1.6.6 milestone but was not fixed until 2 weeks after the 1.6.6 release. Now I have to add a note about this Arduino IDE 1.6.6 bug every time I write Boards Manager install instructions.

Some examples of open issues that I consider blocking:

• Generated prototype for function in tab causes #line directive to have the wrong filename arduino-builder#99
• -mmcu= not passed while generating dependencies arduino-builder#106
• lib manager overrides core libraries included with 3rd party boards #4064

I know I'm probably hoping for too much to expect the last will be done by 1.6.8 but I think all will agree with the first two.