Add mouse click enhancements that don't rely on keyboard selections

[eugenesvk]

(Stacked on top of previous PRs to avoid conflicts)

Finally makethe package a mouse-first citizen! and closing #63

• Add mouse_only command argument so you can have a click that just opens the URL the mouse clicked on, not whatever your cursors selected
  • Add mouse_v_line_affordance on clicks if your mouse cursor is positioned to the right from the end of the current line and at the top/bottom of the current line, then consider it as a click ↑above/below↓
  • Add on_click_ignore_sel on mouse clicks ignore selection ≤ this value and use expansion/scope rules to determine the URL. Helps avoid errors when you click and slightly drag your mouse by ~1 char
  • Add default mouse binding that doesn't drag select

Also adds a couple of helper things

• Logging var to guard debug prints
• Fixes iterating None error, which might've been caused by some of the new features, though not sure exactly which one (though just got it on master, so I guess it's not related)