Added optino to hide/show level # on fold column.

The new option 'foldcolshowlvl' ('fsl' for short), allow one to hide the
numbers vim show on the fold column to indicate the fold level. This can
help use foldcolumn equals one and don't have a crowded fold column.
Added option to configure which characters are used in fold column.

Like 'listchars' you can configure 'foldchars' to define which characters
are used to represent a fold onne fold column. Multi-bytes craters are
allowed.