Make css-compiler optional

[dovydaskukalis]

Currently css compiler is built into core which makes it impossible to create a custom package to handle css imports because of error: conflict: two packages included in my-package (meteor and my-package) are both trying to handle *.css.

Example use case: 3rd party npm_module imports css files like import ./styles.css. I don't want it in my app. My current workaround is cloning meteor package into packages and disabling css compiler plugin while creating a simple package to skip css imports from node_modules. While it works, it feels a bit hacky and I don't think CSS handling should be built into core.

Proposed solution: Split it into it's own package which would be added by default but could be easily removed.