_.size to return symbol count of strings.

Continuing the [discussion](https://github.com/jashkenas/underscore/commit/bdb06bcfcdcc85bfe3b494d9907762945f7d7c49#commitcomment-13243061), at the moment:

``` js
_.size('💕')
// => 2
```

Since `str.length` is trivial I think it --may-- be handy if `_.size` returned the number of symbols in a string instead.

``` js
_.size('💕')
// => 1
```

Start at the _.size entry point and review the linked discussion to understand the intended string-counting behavior. Determine whether “symbol” means code points or another unit, then add coverage for the emoji example and verify the existing collection behavior remains unchanged.

Written by the indexing model from the issue text.