Once initiated mixins support.

Hi jhudson8 -

I added some new functionality to your lib.
Now, this lib supports once initiated mixins.
How it works:

You register mixin:
```
React.mixins.addOnceInitiated('mixinName', mixin);
```
Then, when you try to add this mixin multiple times to your React component:
```
React.createClass({
  mixins: ['mixinName("foo")', 'mixinName("bar")']
  ...
})
```
it will be initiated once and pass all params to your mixin 
```
mixin("foo", "bar")
```
instead of initializing your mixin twice
```
mixin("foo")
mixin("bar")
```

Let me know if you have any questions.
can you define multiple function signatures and keep the parameter definitions to a single line (for https://github.com/jhudson8/fancydocs compliance)

```
#### add (mixinName, mixin[, dependsOn, dependsOn, ...]) or (options, mixin[, dependsOn, dependsOn, ...])
* ***mixinName***: (string) the alias to be used when including the mixin for a React component
* ***options***: (object) {mixinName, onceInitiated}
* ***onceInitiated***: (boolean) defines how parameterized mixins are executed;  see "Mixins With Parameters" for more details
* ***mixin***: the mixin object
* ***dependsOn***: (string or array) the alias of another mixin that must be included if this mixin is included
```

(then could add more details in the "Mixin With Parameters" section please?