Add new g:go_addtags_skip_unexported option

[igrmk]

Tool gomodifytags now has -skip-private option for adding tags only to public fields

This PR mirrors this option in vim config

For example you may have following struct

type Struct struct {
    Public  int
    private int
}

Currently GoAddTags command would add these tags

type Struct struct {
    Public  int `json:"public"`
    private int `json:"private"`
}

With new option it would add

type Struct struct {
    Public  int `json:"public"`
    private int
}

[bhcleek]

Given that :GoAddTags can take a range of line to operate on, what's the value in adding this configuration option that will apply to every invocation?

[igrmk]

I personally never need tags for private fields. So it is just much simpler for me. I don't need to select a range with this option

[bhcleek]

The flag name for gomodifytags has changed. Would you be willing to update this PR similarly?

[igrmk]

PR is updated according to new flag name

[bhcleek]

Thanks @igrmk

Do you mind if I rebase and push a fix for a couple of nitpicks to your branch before I merge?

[igrmk]

Please go ahead! Thanks @bhcleek

[bhcleek]

Thanks for contributing @igrmk !