[feat] allow outDir to work with sub-generators

[fjc0k]

I'm working for my SAO generators.

I want to put all generators in one repo (https://github.com/fjc0k/sao-io) to make maintenance easier.

For example, Mini Program, WebAPP, Cli Tool...

Then, I can use sao io:{mp|h5|cli|lib} [outDir] to start my projects.

But, it's impossible based on the documentation:

Sub-generators are supposed to be running in an existing project, which means the output directory is always current working directory.

So, I hope SAO can allow outDir to work with sub-generators.

THX!