fix postcss config

[shrpne]

nuxt docs says that build.postcss option can be Boolean and Function

I have added condition to check if it's possible to override the postcss option.
Now it will not fail build if user sets postcss to false.

Also, I want to ask, maybe nuxt/press shouldn't set stage to 0? I think this setting is out of scope of nuxt/press module and the user should decide what stage to use.