feature: retain struct tags from definition

[mgutz]

This utility is wonderful! I wasted so much time trying to get Twirp, gRPC working end to end with database marshalling. This utility keeps it simple.

I found myself having to add struct tags to generated structs for use with database packages. Code regen would break things. This PR retains struct tags from the original definition and allows adding/overriding of tags in templates with structTag helpers.

// render existing tags with backticks or EMPTY
<%= struct_tag(field.Tag) %>

// render tags overriding or setting fruit key
<%= struct_tag2(field.Tag, "fruit:\"apple\"") %>

[matryer]

@mgutz Thanks for your contribution. We're working on this a little, we weren't keen on struct_tag2 as a method name, so we're going to go with format_tags inside the template. It takes a variadic function, and works just like your struct_tag and struct_tag2 methods. Pushing shortly.

[matryer]

Thanks for the inspiration @mgutz - release is https://github.com/pacedotdev/oto/releases/tag/v0.6.0.