Standardise github labels

[hadley]

Using https://github.com/hadley/dplyr/labels as a template.

[jimhester]

Maybe we could think about incorporating the Difficulty and Effort labels from https://github.com/rich-iannone/DiagrammeR/labels?

I think they would be useful for potential contributors looking for a way to contribute, or for future planning for ourselves.

[hadley]

I really don't like how makes the issue page look — it's hard to see the issues themselves for all the labels.

[jimhester]

I think they went a little overboard with number of labels and you would need to choose quieter colors, but I think providing some guidance could be very beneficial to possible contributors

[hadley]

Implementation notes:

• Can use gh as it's now on CRAN (need to review other places where this could be used)
• API docs at https://developer.github.com/v3/issues/labels/
• Should create missing labels, and set colours
• Optionally delete default tags?