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/bindings.go

@@ -80,6 +80,8 @@ var bindingActions = map[string]func(*View, bool) bool{
 	"End":                    (*View).End,
 	"PageUp":                 (*View).PageUp,
 	"PageDown":               (*View).PageDown,
+	"SelectPageUp":           (*View).SelectPageUp,
+	"SelectPageDown":         (*View).SelectPageDown,
 	"HalfPageUp":             (*View).HalfPageUp,
 	"HalfPageDown":           (*View).HalfPageDown,
 	"StartOfLine":            (*View).StartOfLine,
@@ -255,6 +257,7 @@ var bindingKeys = map[string]tcell.Key{
 	"Escape":         tcell.KeyEscape,
 	"Enter":          tcell.KeyEnter,
 	"Backspace":      tcell.KeyBackspace2,
+	"OldBackspace":   tcell.KeyBackspace,
 
 	// I renamed these keys to PageUp and PageDown but I don't want to break someone's keybindings
 	"PgUp":   tcell.KeyPgUp,