Add option to disable the shadow.

[WrathChaos]

Hello @marcelglaeser Thank you so much for your contribution.

Actually, I want to remove the whole shadow style from the library. Always a simple Shadow View can be wrapped outside the library. However, this prop is actually redundant because style prop already gives the permission to override the shadow style itself.

Here is the example:

<SearchBar
   style={{ shadowOpacity: 0 }}
   placeholder="Search here..."
   onChangeText={(text) => this.handleFilterList(text)}
   onClearPress={this.handleSearchClear}
 />

Anyway, thank you so much for the contribution tho :) @marcelglaeser