Support "// prettier-ignore" in comment blocks

[ericclemmons]

Pretty much took the example from #1109.

The important caveat is that // prettier-ignore has to be the only or last comment attached to a node.

I could update this to use a .filter(...).length check so it can exist anywhere in the comments, but felt that doesn't really make sense cosmetically to warrant it.

[vjeux]

Thanks, I think it makes sense.

Note that it can be attached as a trailing comment as well.

function x() {
  a(  a  ); // prettier-ignore
  // something else
}

is no longer going to work with your code. So we may want to do comments.some(comment => comment.text === 'prettier-ignore') to be on the safe side, but I'm not really sure if it's worth caring about this edge case. Happy to take another PR for it.

[ericclemmons]

Support for .some looks good enough to use:

http://node.green/#ES2015-built-ins-typed-arrays--TypedArray--prototype-some

I'll send another PR now.

[vjeux]

We already have 7 usages of .some( in the printer :) Also, we run travis on node 4 to make sure it doesn't break for the lowest version of node we support.

[lydell]

(For the record, Array.prototype.some has been around since ES5.)