Add a `clea set` command

[ValentinGot]

The clea set command should allow us to set some global configuration or missing options in the generated projects.

# Use yarn as global package manager to handle projects installation
clea set packageManager=yarn

# Use Angular Material as ui framework
clea set uiFramework=material

# Generate the defaults of a Progressive Web App
clea set makeItProgressive=true

And more ...