Deprecated setUserData / getUserData

[greg0ire]

My browser (firefox) warns against both methods being deprecated and recommends using dataset or Weakmap instead.

[docwhat]

I'm not really interested in maintaining It's All Text! anymore. There are a lot more problems with recent builds than just these issues.

If you know someone interested in taking over I'd appreciate it.

I'd accept a PR if it isn't too hard to review/approve.