Inconsistencies with some of the `org-mode` snippets

[zmberber]

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• First of all, even though it doesn't seem like it in the github UI, it is indeed the case that there is an extra newline at the end of the snippets/org-mode/src snippet. This is annoying.
• Second, the $0 field in snippets/org-mode/src is indented, which is not the case for any of the other snippets.
• Third, the snippets/org-mode/elisp (and snippets/org-mode/emacs-lisp) snippet has a :tangle yes option, which none of the other snippets have. I also don't see why this was chosen anyway, since it is an extra option that either shall be added manually, or if it should be used everywhere, tangling should be enabled by default, and the :tangle no option shall then be used for ones that shouldn't be tangled. I mentioned this in #413 before, where I suggested that one of the two snippets includes it, and the other doesn't.

On the other hand, this isn't that big of a deal since the src, python, and elisp are the only snippets for a src block, so it is easy to override. But this is about having good defaults.