Add shell parameter & add type definitions

[LitoMore]

All to pass in the shell parameter to specify the shell.

[sindresorhus]

Should be an options-object. In case we want more parameters in the future.

[LitoMore]

@sindresorhus Could you share why you delete tsd? I remember that most of your packages use tsd for testing types. Thanks~

[sindresorhus]

I only use tsd when it actually tests something useful. Not for simple types.

[LitoMore]

Got it. Thanks!