Add Modal component

[jhen0409]

This patch solves the following problem

#91 Modal component

• The shortAnimTime is referenced from RN for Android anim duration, that used for modal. (config_shortAnimTime: 200)
• Referenced official documentation (havn't add deprecated animated prop and platform only props)

Test plan

• Simple snapshot tests for visible prop. index-test.js.snap
• Added examples to storybook, deployed on my fork

This pull request

• includes documentation
• includes tests
• includes benchmark reports
• includes an interactive example
• includes screenshots/videos

[ndbroadbent]

Oh awesome, I was just looking for this so that I could get react-native-material-dialog working on web.

I would love to help test this, but it looks like there's a conflict that needs to be fixed.

[necolas]

I think the animations could be implemented as CSS transitions (for performance).

While this is a good start, I think the Modal implementation will be more complicated than this as it needs to be rendered outside the tree of your main app (i.e., probably requires working into the AppRegistry root view) in order to get the stacking correct and to support launching modals from within any layer context (e.g., a ScrollView or anything else using translate3d)

[jhen0409]

Move to #471.

Sorry I haven't so much time for look this. :|