Add method to prefix following calls to transport

Added method to solve the problem of needing to prefix calls to transport with another function. You can now do this:

```
transport.chdir(path, function() {
  transport.git('pull');
  transport.npm('install');
});
```

which would be the same as calling

```
transport.exec('cd my-path && git pull');
transport.exec('cd my-path && npm install');
```

Fixes pstadler/flightplan#2