add special command '%cd [path]'

Author: cosmos72

README.md

@@ -200,11 +200,19 @@ $ docker run -it -p 8888:8888 -v /path/to/local/notebooks:/path/to/notebooks/in/
 
 - Have fun!
 
+## Special commands
+
+In addition to Go code, the following special commands are also supported - they must be on a line by their own:
+- %cd [path]
+- %go111module {on|off}
+- %help
+- $ shell_command [args...]
+
 ## Limitations
 
 gophernotes uses [gomacro](https://github.com/cosmos72/gomacro) under the hood to evaluate Go code interactively. You can evaluate most any Go code with gomacro, but there are some limitations, which are discussed in further detail [here](https://github.com/cosmos72/gomacro#current-status).  Most notably, gophernotes does NOT support:
 
-- third party packages when running natively on Windows - This is a current limitation of the Go `plugin` package.
+- importing third party packages when running natively on Windows - This is a current limitation of the Go `plugin` package.
 - some corner cases on interpreted interfaces, as interface -> interface type switch and type assertion, are not implemented yet.
 - some corner cases on recursive types may not work correctly.
 - conversion from typed constant to interpreted interface is not implemented. Workaround: assign the constant to a variable, then convert the variable to the interpreted interface type.

kernel.go

@@ -719,8 +719,9 @@ func evalSpecialCommands(ir *interp.Interp, outerr OutErr, code string) string {
 func evalSpecialCommand(ir *interp.Interp, outerr OutErr, line string) {
 	const help string = `
 available special commands (%):
-%help
+%cd [path]
 %go111module {on|off}
+%help
 
 execute shell commands ($): $command [args...]
 example:
@@ -734,7 +735,18 @@ $ls -l
 		arg = args[1]
 	}
 	switch cmd {
-
+	case "%cd":
+		if arg == "" {
+			home, err := os.UserHomeDir()
+			if err != nil {
+				panic(fmt.Errorf("error getting user home directory: %v", err))
+			}
+			arg = home
+		}
+		err := os.Chdir(arg)
+		if err != nil {
+			panic(fmt.Errorf("error setting current directory to %q: %v", arg, err))
+		}
 	case "%go111module":
 		if arg == "on" {
 			ir.Comp.CompGlobals.Options |= base.OptModuleImport