Support and option to allow underscores in integers

[ingydotnet]

Some 1.1 emitters put underscores in integers.

They are not part of the 1.2 spec schemas, but it would help compatability to be able to support this and other cases.

With the addition of a proposed config object we could allow this either with a specific option or the ability to set the pattern to match for the !!int tag.

[ingydotnet]

I realized pyyaml does allow underscores in integers.

In 6.1 we will likely support 1.1 and 1.2 schemas, but it still makes sense to support a compatibility option to allow them in 1.2 !!int.