Add alternative execute notation that does not require closing char

This can be used to avoid parse errors that can happen when the command
contains the closing character. Since the command does not finish at
a certain character, the key binding should be the last one in the
group. Suggested by @tiziano88. (#265)

  e.g. fzf --bind "ctrl-m:execute=COMMAND..." --bind ctrl-j:accept

src/options_test.go

@@ -163,7 +163,8 @@ func TestBind(t *testing.T) {
 		parseKeymap(keymap, execmap, false,
 			"ctrl-a:kill-line,ctrl-b:toggle-sort,c:page-up,alt-z:page-down,"+
 				"f1:execute(ls {}),f2:execute/echo {}, {}, {}/,f3:execute[echo '({})'],f4:execute:less {}:,"+
-				"alt-a:execute@echo (,),[,],/,:,;,%,{}@,alt-b:execute;echo (,),[,],/,:,@,%,{};")
+				"alt-a:execute@echo (,),[,],/,:,;,%,{}@,alt-b:execute;echo (,),[,],/,:,@,%,{};"+
+				",X:execute=foobar,Y:execute(baz)")
 	if !toggleSort {
 		t.Errorf("toggleSort not set")
 	}
@@ -181,6 +182,7 @@ func TestBind(t *testing.T) {
 	checkString("less {}", execmap[curses.F4])
 	checkString("echo (,),[,],/,:,;,%,{}", execmap[curses.AltA])
 	checkString("echo (,),[,],/,:,@,%,{}", execmap[curses.AltB])
+	checkString("foobar,Y:execute(baz)", execmap[curses.AltZ+'X'])
 
 	for idx, char := range []rune{'~', '!', '@', '#', '$', '%', '^', '&', '*', '|', ':', ';', '/'} {
 		keymap, execmap, toggleSort =