Allow \ or , as leader key in vim.

This helps when working with some people who are used to using the
deafult leader key (\) and others who have remapped it to the more
common setting (,).

From http://stackoverflow.com/a/8397808
Make shifted cursor keys work in vim, to select text.

This includes mapping them from Mac Terminal and XTerm key-bindings to
vim bindings, and having the vim bindings extend the visual mode
selection.