Allow recursion using `,recurse`

[SilverCory]

This allows sql:"..." to recurse into the struct tagged.

[kisielk]

Hm, wouldn't just ".." be more familiar looking?

[SilverCory]

I was going along the lines of func(t ...T), if I saw .. I would thing up in the hierarchy not down

[MMulthaupt]

But this isn't about varargs. If it needs to be any amount of dots, I believe a single . makes most sense since you are accessing a member of the struct, going down one level in the hierarchy, which is exactly what it looks like elsewhere in the language.

A different idea: add another tag-field. Kinda how for json we may write json:",omitempty" we could write sql:",recurse".