Add -r for printing method receivers

[segevfiner]

Please consider if the format I'm printing these is OK:

foo.go:

package main

type Foo struct{}

func (f *Foo) bar() {

}

Running godef:

$ godef -t -r -f foo.go -o 52
foo.go:5:15
bar (*Foo) func()

Fixes #93
See: microsoft/vscode-go#2107

[segevfiner]

@ramya-rao-a I will also be glad to hear what you think as we will have to parse this in vscode-go.

[ramya-rao-a]

The output looks good to me.

cc @rogpeppe @ianthehat

[segevfiner]

ping @rogpeppe @ianthehat