Move key bindings into custom autocmd group

Vim is smart enough to map key bindings itself. No need to call
`s:SetupMappings` on each load.
Expose <Plug> mappings for useful commands
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Update documentation

Add refs to <Plug> mappings. Almost all commands is used in normal mode so
keep mode column is too verbose.
Remove duplicate q and u mappings

* q mapped to <Plug>(gitv-quit)
* u mapped to <Plug>(gitv-redraw)
Merge pull request #1 from vitalk/support-mappings-customization

Support mappings customization