feat(SearchBox): add placeholder props to the underlying input element
feat(SearchBox): add placeholder props to the underlying input element

* Original commit: 399410679229999dca027602a9a5d97b94b596de
* Original author: @Heedster
* Closes #115
* Closes #136

tomchentw/react-google-maps,jossi87/react-google-maps,ksavenkov/react-google-maps,RishabhJain96/react-google-maps,jossi87/react-google-maps,joshcarr/react-google-maps,RishabhJain96/react-google-maps