push-stack support

[furiel]

This little patch adds a new interactive function to the api: helm-gtags-push-stack.

My motivation is I can typically jump around the code with the current api, but in some cases I use helm-grep-do-git-grep for jump, for example when I do not exactly know what is the tag I am looking for, or when I am looking for a complex expression. Even then my muscle memory triggers the helm-gtags-pop-stack, which jumps to a wrong location, as helm-grep-do-git-grep does not push location to the stack. With helm-gtags-push-stack, I could advice-add helm-grep-do-git-grep to push the location before search, letting me to continue to use helm-gtags-pop-stack safely.

In general, this function can be used to make other modules compatible with helm-gtags.

[furiel]

@syohex can you look into this? I would be happy to see helm-gtags-push-stack available in melpa.

[syohex]

Thanks for PR