Signed-off-by: yuexin <yuecn41@gmail.com>

Fix #99.

Vim already has a well known way of dealing with this:
let's not create another configuration which only raises
the entry barrier.

Deleted overriding global fold configuration for markdown buffers and
improved README on purpose of vim_markdown_folding_disabled.

Vundle is changing their interface from `Bundle*` to `Plugin*`.
See 
https://github.com/gmarik/Vundle.vim/blob/v0.10.2/doc/vundle.txt#L372-L396 for more information on this.

Every non local identifier must start with g:vim_markdown_ from
CONTRIBUTING.md.

Also updated the readme with information about the command.