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/cellview.go

@@ -76,8 +76,8 @@ func (c *CellView) Draw(buf *Buffer, top, height, left, width int) {
 			curX := buf.Cursor.X
 			curLoc := buf.Cursor.Loc
 			if buf.Settings["matchbraceleft"].(bool) {
-				curX--
 				if curX > 0 {
+					curX--
 					curLoc = curLoc.Move(-1, buf)
 				}
 			}