Aded function-es6-concise method based on @gaearon's c963c0b47452124137fe30cc74a52799a25adc0d

Using a lookahead assertion to match the function signature and opening brace, allowing the function parameters and body to be handled as per the other function patterns
Added support for namespaced JSX based on @roadhump's aa6b558f2a09aec1266c993e5424414d881e3b7e