enhancement with section header char for locating and control options about the ruler

[charlie-wong]

• add section header char, thus the section can be located
  • make diff-so-fancy works together with ov with more pertty output
  • add two options, -use-sc DIFF for customization, -N to reset/clean it
• add options to control the ruler type By default, top/bottom ruler is showing with unicode line-drawing chars. And if a ruler is marked to hiding, by default it will be replaced by newline.
  • -U mark top ruler to hiding, and -u make top ruler to showing
  • -D mark bottom ruler to hiding, and -d make bottom ruler to showing
  • -X mark showing ruler to be replaced with newline
  • -x mark hiding ruler to be replaced with newline

More examples and details can be found in README.md