Simplify configuration of temporary files

Revert 1072cfd325bbc6869ad54b2c5228c4af720d5a74 and update
f9205bfa61077ff1581e2acc9030dd38f1678a26 in order to have a simple
configuration.

In short:

- Don't use different cache directories for Linux and OS X.
- Put swap, backup and undo files in the same directory
  (backup is disabled, but set its location anyway for completeness).
- Prefer ~/.cache/vim//, otherwise fall back to Vim's defaults.
- The double // indicates to use the complete path to name the files.
install.sh: check and use ~/.vim as working dir
README: replace '$HOME' with '~'
Highlight search

Otherwise, what's the point of the nice <C-l> remap to clear the
highlights  a773e76da79093988160b8097396c7cb548a32ac
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
Big cleanup of ftplugin files: remove comments

- Remove folding (its just a bunch of short files, no need to annoyingly
  fold a few lines)
- Remove option comments (unless it is a rare one)
Big cleanup of ftplugin files: options

- When setting indentation width, always set the three related values:
  tabstop, softtabstop and shiftwidth [1].
- Remove options that replicate the default value since I will not
  change it (tw=0, ts=4, sw=4, nospell, linebreak).

[1]: http://tedlogan.com/techblog3.html
Use default formatoptions

Use the Vim distribution values for each filetype, but set it for those
filetypes that doesn't change the Vim default value (e.g. python).

Note: formatting the paragraph using the indentation of the second line
is now removed of the options. I need to figure out in which cases I was
using it to restore it for every filetype individually.
vimrc: small reordering of options

For better logical grouping.
Sync my options with vim-sensible plugin

- Don't consider octal numbers using <C-a> and <C-x>.
- Adjust timeouts for keycodes.
- Search tags upwards if not found.
- Discard options when saving sessions (the session can be corrupted).