a few miscellaneous fixes and improvements (#1105)

* add binding for more primitive backspace

* support selecting page up and page down

* fix matchbraceleft for braces that start on x=0

* fix multiline copy-paste indenting

let's say you have two lines like

  <space><space>line1
  <space><space>line2

so you start from cursor x=0 and select both lines, then paste.
we don't want any leading whitespace in this case, because the
cursor is already at x=0 and the selection already includes
whitespace.

cmd/micro/view.go

@@ -198,7 +198,10 @@ func (v *View) ToggleTabbar() {
 }
 
 func (v *View) paste(clip string) {
-	leadingWS := GetLeadingWhitespace(v.Buf.Line(v.Cursor.Y))
+	leadingWS := ""
+	if v.Cursor.X > 0 {
+		leadingWS = GetLeadingWhitespace(v.Buf.Line(v.Cursor.Y))
+	}
 
 	if v.Cursor.HasSelection() {
 		v.Cursor.DeleteSelection()