Allow for custom search paths for iOS framework dependencies

If one wishes to use a custom framework dependency (such as from a 3rdparty), the build will fail because the linker will not know where to look for it.

This change, along with openfl/lime#155 fixes this by allowing the user to specify a custom path for the framework dependency as follows:
```
<dependency path="libs/VungleSDK.embeddedframework/" name="VungleSDK.framework" />
```
The path is where the framework is located and name is the name of the framework.