adds min/max empty lines after document start/end

[wookietreiber]

@adrienverge What do you think?

Note: This currently only includes the changes to document start. Will do the changes for document end and documentation later, based on your feedback.

---

fixes https://github.com/adrienverge/yamllint/issues/340

[coveralls]

Coverage increased (+0.02%) to 97.528% when pulling 4c86d3fe5930b123375c96c479a0d41526e63d03 on idiv-biodiversity:document/empty-lines into 43744902e90fb33905f1000875c60acd06661f2c on adrienverge:master.

[wookietreiber]

@adrienverge Thanks for the review. I'm gonna do the rest of the work, likely spread over the next couple of days. When I'm done, I'm going to remove the draft status and ping you for another review (dunno if removing the draft issues a notification, maybe re-requesting a review does).