Add -s flag for gofmt parity

[johanbrandhorst]

gofmt offers a -s parameter for simplifying code, which is quite nice. I think goreturns should add this parameter as well since it promises to support a superset of gofmt's features, and I'd like both simplified code and auto-returns.

[sqs]

Yes, this would be great. Overall I would like to overhaul goreturns soon and make it shell out to goimports/gofmt and (probably) pass along flags to future-proof it.

A PR for this would be gladly accepted to get this in sooner, though!

[MaerF0x0]

is this still the planned path? I can see if I can handle -s