Improve preservation of position when switching buffers

Solution in 95fec4f42e does't work in this use case:

- Open a buffer in window A, keep cursor in line L1
- Open the same buffer in window B, keep cursor in line L2
- On window B switch to another buffer
- Switch to window A

The position in window A will jump to L2, instead of staying on L1.

This new autocommands will preserve position based on both buffer and
window.

[1]: http://vim.wikia.com/wiki/Avoid_scrolling_when_switch_buffers
Replace vim-powerline with vim-airline

lean & mean status/tabline for vim that's light as air

https://github.com/bling/vim-airline
Don't display fancy symbols on vim-airline

I don't want to deal with patched fonts :)
Add vinegar plugin

Combine with netrw to create a delicious salad dressing

https://github.com/tpope/vim-vinegar
Change mappings to switch between buffers
Support bundle and vimrc files of a fork

So, the vimrc will try to load the fork files first and then the local
files.