History file handle leak

The history file is not closed when it is reset, resulting in leaked file handles.

Here's a program that demonstrates the bug:
```
package main

import "gopkg.in/abiosoft/ishell.v2"

func main() {
shell := ishell.New()

shell.AddCmd(&ishell.Cmd{
Name: "on",
Func: func(ctx *ishell.Context) {
shell.SetHistoryPath("test-history")
},
})
shell.AddCmd(&ishell.Cmd{
Name: "off",
Func: func(ctx *ishell.Context) {
shell.SetHistoryPath("")
},
})

shell.Run()
}
```

By watching the output of `lsof -c main` on *nix can see that every call to `on` leaves the previous history file handle open. `off` does not close the existing file handle either.

It seems you need something like:
```
diff --git a/ishell.go b/ishell.go
index 098af06..c0545c1 100644
--- a/ishell.go
+++ b/ishell.go
@@ -393,6 +393,7 @@ func (s *Shell) SetHistoryPath(path string) {
// Instance.
config := s.reader.scanner.Config.Clone()
config.HistoryFile = path
+ s.reader.scanner.Close()
s.reader.scanner, _ = readline.NewEx(config)
}
```