org-git-link.el: Add support for line in org-git-link

* org-git-link.el (org-git-open): Jump to the line if provided in the link.
(org-git-split-string): Parse a third element of the list and change the error
message accordingly.
(org-git-create-git-link): Accept the third optional line argument.
(org-git-store-link): call `org-git-create-git-link' with the current line
number.