[Bug] checkbox markers not set correctly

[mstojanovic]

by my understanding of the doc:
let g:bullets_checkbox_markers = ' .:oOX'
should work: 0% ' ', <0%-25%> '.', [25%-50%> ':', [50%-75%> 'o', [75%-100%> 'O', 100% 'X'

on a list with 6 subitems the top level gets marked with a 'X' after 5 are checked in the sublist

[mstojanovic]

created pull request #101

edit: percentage limits are as described above