add no_omitempty option to remote omitempty label from generated struct

[sparkthu]

For #682
add an new option "no_omitempty", which is default false to keep default appearance.
when call go_out=no_omitempty=true:./out.go, structs without omit_empty label for json will be generated.

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[sparkthu]

I signed it!

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[sparkthu]

I signed it!