Add yaml 1.1 octal parsing behavior

[firstdorsal]

See #17

[acatton]

Thank you for this pull request. There is a test and everything.

I'm just concerned about breaking things for people using this create, especially since we want to support 1.2 in the future (see #5 )

Let me sleep over this and think how to fix that. At the top of my head, I'm thinking about merging your pull request, but guard it behind a flag, like no1.2compatibilty or something like that.