Added root view style

[diegolmello]

Currently, root view has no style applied.
With this PR, it's possible to do it through view style.

Example:

import Markdown, { AstRenderer, renderRules, styles } from 'react-native-markdown-renderer';

const renderer = new AstRenderer({
    ...renderRules
}, {
    ...styles,
    view: {
        flex: 1,
        backgroundColor: 'blue'
    }
});

return <Markdown renderer={renderer.render}>Text</Markdown>

[mientjan]

@diegolmello yes, you are right we should add this but i think the style should not be view but root. see c6dd684 the change i made. thx for the pull.