Support custom activities

[grabbou]

This pull request adds ability to include custom app specific activities that subclass UIActivity.

To use it, subclass UIActivity (make custom one) and make it react module, example code:

// InstagramActivity.h
#import "RCTBridgeModule.h"
#import <UIKit/UIActivity.h>

@interface InstagramActivity : UIActivity <RCTBridgeModule>

@end

// InstagramActivity.m
#import "InstagramActivity.h"
@import UIKit;

@implementation InstagramActivity

@synthesize bridge = _bridge;

RCT_EXPORT_MODULE(InstagramActivity)

- (NSString *)activityType
{
    return @"com.grabbou.whatever";
}

- (NSString *)activityTitle {
    return @"Custom App";
}

- (UIImage *)activityImage {
    return [UIImage imageNamed:@"Logo"];
}

- (BOOL)canPerformWithActivityItems:(NSArray *)activityItems {
    return YES;
}

- (void)prepareWithActivityItems:(NSArray *)activityItems {
}

- (UIViewController *)activityViewController {
    return nil;
}

- (void)performActivity {
    [self activityDidFinish:YES];
}

@end

And just call:

ActivityView.show({
   customActivities: ['InstagramActivity']
});

which more or less produces the below result:

---

@naoufal EDIT: Closes #28

[naoufal]

Let's rename this to customActivitiesToInclude.

[naoufal]

@grabbou Could you update the README to reflect your changes?

[naoufal]

Aside from the couple comments, LGTM 👍