Adds ability to do Hoek.reach with negative index

Fixes #114

So I did benchmark this with three different versions.

Version 1 was plain without negative index.
Version 2 used an initial check to see if there's any negative in the string, plus a negative for each part.
Version 3 just does a check for a negative for each part of the path (v2 in test)


The result was this: 

```
get with index x 3,005,799 ops/sec ±0.24% (1094 runs sampled)
get with negative index x 2,180,143 ops/sec ±0.21% (1091 runs sampled)
get with negative index v2 x 2,427,552 ops/sec ±0.25% (1094 runs sampled)
Fastest is get with index
      386.24 real       180.59 user         1.34 sys
```


See below for the code for the test. let me know if you'd like me to test it some other way, but I thought with 1000 samples something would have come up if it was significantly slower.

``` javascript

var Hoek = require('hoek');
var HoekNeg = require('hoek-neg');
var HoekNeg2 = require('hoek-neg2');
var Benchmark = require('benchmark');
var suite = new Benchmark.Suite;

var a = [1, 2, 3, 4, 5, 6, 7, 8];
suite.add('get with index', function () {


    var b = Hoek.reach(a, '0');
})

suite.add('get with negative index', function () {


    var b = HoekNeg.reach(a, '0');
})

suite.add('get with negative index v2', function () {


    var b = HoekNeg2.reach(a, '0');
})


suite.on('complete', function () {
    console.log('Fastest is ' + this.filter('fastest').pluck('name'));
});

suite.on('cycle', function (event) {
    console.log(String(event.target));
})

suite.run({ async: true, minSamples: 150, name: 'whatever'  });

```
I think `key[0] === '-'` would be faster, no need to go through the whole string if you only want the 1st letter.