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

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.

Also updated the readme with information about the command.

Syntax-based folding was way too slow in very large Markdown files. Add
a new file after/ftplugin/mkd.vim with a foldexpr function to determine
folds. The function is based on Steve Losh's gist at
https://gist.github.com/1038710