Introduce base commands.

Base commands are useful when creating templates.
Real life example:

```
var app = function() {
  return nixt({ newlines: false }).cwd(bin).base('./curiosity ').clone();
};

it('returns the app version', function(done) {
  app()
  .stdout(require('../package.json').version)
  .run('--version')
  .end(done);
});
```
Merge branch 'base-cmd'

* base-cmd:
  Introduce base commands.