vim: switch <tab> mapping to <s-tab>
vim: switch <tab> mapping to <s-tab>

Sigh, not as convenient, but Vim won't allow you to map `<tab>` and
`<c-i>` independently as discussed here:

- http://stackoverflow.com/questions/14641942/how-to-unmap-tab-and-do-not-make-ctrl-i-invalid-in-vim
- http://vim.1045645.n5.nabble.com/how-to-map-lt-tab-gt-and-lt-c-i-gt-independently-td1185417.html
- https://groups.google.com/d/msg/vim_dev/Ym6D-kWIsyo/-XOw7E8LbN0J
- http://catern.com/posts/terminal_quirks.html
- http://www.leonerd.org.uk/hacks/fixterms/

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