Bind Fold#headOption method to this

[mlegenhausen]

Allows the usage of the headOption method when used as reference for example in a pipe statement. Without the bind the this context gets lost and an error like this occurs:

Uncaught TypeError: Cannot read property 'find' of undefined
  at push.../../node_modules/monocle-ts/es6/index.js.Fold.headOption

[felschr]

This should fix #96.

I'm frequently running into this issue. Here is a common example:

pipe(
  [1, 2, 3, 4, 5],
  fromTraversable(Arr.Traversable)<number>().asFold().headOption,
)

Wrapping the headOption call in an arrow function can be used as a workaround:

pipe(
  [1, 2, 3, 4, 5],
  (a) => fromTraversable(Arr.Traversable)<number>().asFold().headOption(a),
)