Issue title: Input Invariant Violation
 Issue body: ### Description

1. Updated from beta2 to beta3
2. Input doesn't crush the app
3. Got console error

```
Invariant Violation: Element type is invalid: expected a string (for built-in components) or a class/function (for composite components) but got: undefined. You likely forgot to export your component from the file it's defined in, or you might have mixed up default and named imports
```

### Reproduction Steps and Sample Code
Import `Input` as in docs.


 Comments: 
 Comment 0: Hey @bohdanbirdie, you're right this is a bug, we also set the background to the default blue primary color. The problem is [this line](https://github.com/react-native-training/react-native-elements/blob/master/src/buttons/ButtonGroup.js#L97) if someone wants to tackle this 💪 
 Comment 1: I would like to work on this. Will make a pr as soon as I can
 Comment 2: @k3ithl1m If you hadn't had a chance to work on this issue, like to fix it.
 Comment 3: Hey @dincozdemir Sure thing. I forgot about this issue entirely as I was traveling. Go at it and have fun! :)
 Comment 4: While i was fixing the issue, I also noticed that this usage gives an error when the given style is defined using ``
StyleSheet.create``:

``borderRightColor:
                  (innerBorderStyle && innerBorderStyle.color) || colors.grey4,``

That's because in such case, innerBorderStyle is a style id, not the style itself.
It should be ``StyleSheet.flatten(innerBorderStyle)`` to avoid crashes.

I see this usage pattern through all the components. I wonder if this is a known issue.
 Comment 5: @dincozdemir As you can see [in the docs](https://react-native-training.github.io/react-native-elements/docs/button_group.html#buttongroup-props), the `innerBorderStyle` has the type `object { width, color }`, it cannot be a Stylesheet object, it's basically a simple object that allows you to override these two border props. Other styles has the type `object (style)`, which can be an object OR a Stylesheet object. The name is a bit confusing tho
{'source': 'GithubIssues', 'repo_name_with_owner': 'react-native-training/react-native-elements', 'issue_id': 306172956, 'repo_stars': 12824.0}