Syntax highlight support for markdown preview #21

[yu1l]

Hi !
This pull request adds syntax highlight support for markdown preview. - realate to #21
I used react-native-syntax-highlighter for it.

// in app/views/NoteModal.js
...
import NotePreview from '../components/NotePreview'
...
<NotePreview
    style={styles.container}
    theme={"github"}
    syntaxFontSize={14}
    syntaxFontFamily={Platform.OS === 'ios' ? 'Menlo-Regular' : 'monospace'}
    text={this.state.text} />

I tried to find which theme suits boostnote-mobile, but couldn't find.
So I just applied github theme. theme list is here