split and merge 'piles' of items

A data structure for groups of identical objects, up to a maximum number. Useful for games. (This module was previously known as "itemstack".)

Can be used standalone but most useful with inventory.

Requires a ES6-compatible environment (at least partially), tested on Node v4.2.4

An item pile can be created simply with an item name and count, for example:

```
var ItemPile = require('itempile');
var x = new ItemPile('dirt', 10);
```

represents a quantity of 10 dirt. The item type can be an any comparable object (singleton); these examples use strings. The quantity can be omitted to use a default of "1".

Piles of the same type can be merged:

```
var a = new ItemPile('dirt', 10);
var b = new ItemPile('dirt', 20);
a.mergePile(b);
```

results in `a`

increasing to 30 and `b`

to 0. `mergePile`

returns `false`

if the piles differ
in type and cannot be merged, otherwise the number of items that did not fit (excess above
the maximum pile size):

```
var a = new ItemPile('dirt', 1);
var b = new ItemPile('dirt', 80);
a.mergePile(b);
```

increases the count of `a`

to 64, the default `ItemPile.maxStackSize`

limit, and decreases `b`

to 17.
The sum of the two pile counts remains invariant, the quantity has just shifted between the two.

Want to take items from a pile? Split the pile, specifying the number of items you want:

```
var a = new ItemPile('dirt', 64)
var b = a.splitPile(16)
```

`b`

is a new pile with 16 dirt, `a`

is lowered to the remaining 48 dirt. For convenience you can alternatively pass a
decimal fraction (such as 0.5, splits the pile in half), or a negative integer (-1 to take all but one).

Merging/splitting are the most important but several other methods are provided, see the unit tests for further examples.

You can create piles of infinite size:

```
new ItemPile('diamond', Infinity)
```

and they behave as you expect, sinking unlimited items when merging and sourcing unlimited items when splitting.

Extra data can be attached to a pile, using the "tags" parameter:

```
new ItemPile('pick', 1, {damage:0})
```

MIT