Some updates to the vim configuration

- Added powerline
- Added some customization like show line numbers and cursorline highlight colors
- Added some useful maps to work with windows
- Added some shortcuts to go to the end/beginning of the line in insert mode
- Added .gvim configuration for MacVim
- Added Dracula theme for MacVim
Change the ZSH theme to agnoster

In order to remove the username from the prompt, the DEFAULT_USER must be set.