1. Add a specific vbookmark_sign color
2. Fix a problem that cannot toggle bookmark if there is already sign in current line
3. New function "s:Vbookmark_findNextMarkInFile()"
   (maybe low performance to linear search for all marks)
4. New function "s:Vbookmark_clearMarkInFile()" to clear all marks in current file
5. New commands "VbookmarkFindNextMarkInFile" and "VbookmarkClearMarkInFile"