Vim and MacVim +python3/dyn fix

Fixes for issue #31337:

Vim and MacVim now only *either* support python **or** python3, not both (and, in particular, not the dynamically loaded versions that lead to segfaults in the first place). If either formula is brewed `--with-python3`, python[2] support is automatically disabled. For MacVim, unless `--without-python` is explicitly defined, this shows a Caveat reminding the user that only one version of Python is supported. With Vim, the issue that only one Python version can be supported is made explicit in the list of options available.

Last, while updating the two formulae, I noticed that while the MacVim formula supports Perl "out of the box", Vim's formula does not. As this is confusing, now the Vim formula is adapted to work just as MacVim, with Perl support "on" by default.
The comment documents non-obvious behavior: contrary to intuition, where
you would expect that putting the line *after* the definition of the same
option to overwrite it, it is necessary to put it *before*.
On Aug 8, 2014 11:19 PM, "Jack Nagel" <notifications@github.com> wrote:

> In Library/Formula/vim.rb:
>
> >
> > +  # override definition of python3 support
>
> this comment isn't necessary
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/Homebrew/homebrew/pull/31367/files#r16017129>.
>