Access flag defaults

[mattes]

Right now it's not possible to access flag defaults like /bin/bash from flag shell. As a work-around commands have to manually check for empty values and set the default again. This is redundant and should be changed.

FuguFlags["shell"].String([]string{"-shell"}, "/bin/bash", "Path to shell")

see https://github.com/mattes/fugu/blob/v1/fuguflags.go#L43 and https://github.com/mattes/fugu/blob/v1/fugu.go#L158