indentLine will overwrite user's "conceallevel" and "concealcursor" value by default. It's convenient if users didn't set any conceal value in their dotfile or plugin.
If they set custom conceal value, it confuse them when their setting didn't work. So maybe we should provide an option to keep their setting.
add option: g:indentLine_setConceal
add g:indentLine_setConceal detail to doc and readme