Add YouCompleteMe plugin.

The YouCompleteMe plugin allows auto-completion of source code based on
semantic completors (Clang for C/C++/Objective-C) and normal Omnifunc methods.
Added indent guides plugin.
Change listchar tab character.
Add custom options for markdown files.
Remove set hidden option.

For some reason this causes a lot of file already open errors when working with
several buffers.
Remove foldlevelstart option.

This causes a lot of lag when editing large latex files.
Persist undo history between vim invocations.
Add new colorscheme
Add slim templates plugin.
Remove java-api plugin and replace javacomplete plugin with newer version.
Enable and configure preview plugin.
Add NERDTree ignore list and don't close it when opening a file.
Make the statusline setting more flexible.