only get closes

A common situation is that one just wants the close (or adjusted close) from a set of tickers. It would be nice if getSymbols or some other function could directly output an xts object with one column per symbol.

Start by reading the getSymbols entry point and its documentation, focusing on how a set of tickers is returned. Determine how the close or adjusted close series could be selected into an xts object with one column per symbol. Done means the requested output works for multiple tickers and both close choices are clear to users.

Written by the indexing model from the issue text.