An unsung byproduct of oil and natural-gas production is getting more attention from hedge funds and investors as they seek new ways to bet on the U.S. shale boom.  Natural-gas liquids, which include ethane, propane, butane, isobutane and natural gasoline, are separated out from crude oil and natural gas. They are known as the fuels in propane grills and butane lighters and as feedstocks to make plastics and chemicals. Until recently, they attracted little investor consideration.