Fold columns configuration

I added two options to allow user to configure the way they want their fold column.

The first one is to not print the number representing the fold level which is very annoying when you have foldcolumn=1 or 2.

The second one is to define which characters are used to represent the fold column.

The combination of both can give me the image attached:

![vim_folds](https://f.cloud.github.com/assets/1261/164701/0780ad5a-78d7-11e2-94e5-ab63ca7235a6.jpg)