add clear command to clear file selection

Mentioned in #45.

comp.go

@@ -36,6 +36,7 @@ var (
 		"yank",
 		"delete",
 		"put",
+		"clear",
 		"renew",
 		"sync",
 		"echo",

doc.go

@@ -32,6 +32,7 @@ The following commands are provided by lf with default keybindings.
     toggle            (default "<space>")
     invert            (default "v")
     yank              (default "y")
+    clear             (default "c")
     delete            (default "d")
     put               (default "p")
     renew             (default "<c-l>")

docstring.go

@@ -36,6 +36,7 @@ The following commands are provided by lf with default keybindings.
     toggle            (default "<space>")
     invert            (default "v")
     yank              (default "y")
+    clear             (default "c")
     delete            (default "d")
     put               (default "p")
     renew             (default "<c-l>")

eval.go

@@ -303,6 +303,18 @@ func (e *callExpr) eval(app *app, args []string) {
 		app.nav.save(false)
 		app.nav.saves = make(map[string]bool)
 		saveFiles(nil, false)
+	case "clear":
+		if err := saveFiles(nil, false); err != nil {
+			msg := fmt.Sprintf("clear: %s", err)
+			app.ui.message = msg
+			log.Printf(msg)
+			return
+		}
+		if err := sendRemote("send sync"); err != nil {
+			msg := fmt.Sprintf("clear: %s", err)
+			app.ui.message = msg
+			log.Printf(msg)
+		}
 	case "renew":
 		app.ui.sync()
 		app.ui.renew()

opts.go

@@ -58,6 +58,7 @@ func init() {
 	gOpts.keys["v"] = &callExpr{"invert", nil}
 	gOpts.keys["y"] = &callExpr{"yank", nil}
 	gOpts.keys["d"] = &callExpr{"delete", nil}
+	gOpts.keys["c"] = &callExpr{"clear", nil}
 	gOpts.keys["p"] = &callExpr{"put", nil}
 	gOpts.keys["<c-l>"] = &callExpr{"renew", nil}