Add support for abstract class properties

[soda0289]

What version of TypeScript are you using?
2.3

What version of typescript-eslint-parser are you using?
2.1

What code were you trying to parse?

abstract class Foo {
  abstract bar;
  abstract baz = 3;
}

What did you expect to happen?
Bar and baz properties should be labeled as abstract.

What happened?
They use the same node type as class properties