Add option to specify default plugin options (closes #11)

[rexxars]

As outlined in #11, it would be useful to be able to provide default options for plugins.

This implements that. I couldn't see any good way to merge non-object options, such as arrays - it's unclear whether it's meant to override or merge them when this is the case.

[coveralls]

Coverage remained the same at 100.0% when pulling 2e3920eef172f0922ce5b9e6b2386d39a2aa3b24 on sanity-io:default-plugin-options into abe815d6dee0004e6d8e924a8ad68285a7e77a00 on postcss:master.

[rexxars]

Sorry, I didn't test this under Node 0.12. Added some code to work around Object.assign not being present there.

[coveralls]

Coverage remained the same at 100.0% when pulling 0f4e97f4de354f55b9e6c3f84d4a09767448a801 on sanity-io:default-plugin-options into abe815d6dee0004e6d8e924a8ad68285a7e77a00 on postcss:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 0f4e97f4de354f55b9e6c3f84d4a09767448a801 on sanity-io:default-plugin-options into abe815d6dee0004e6d8e924a8ad68285a7e77a00 on postcss:master.

[ben-eb]

Hi @rexxars, thanks for the pull request!

Do we need the isPlainObject checking? Any time that I've been using options objects, it's always been obvious that we should supply a plain object, that is pretty much a standard pattern across most if not all JS modules.

Also, I think it would be nice if you could remove the polyfill and let Babel polyfill it for 0.12, via object spread. So we can get rid of a lot of code by swapping out;

return assign({}, defaultOpts, specifiedOpts);

with

return {
  ...defaultOpts,
  ...specifiedOpts,
};

Also, defaultPluginOptions is a little verbose. Perhaps simply options is a better name for this?

[rexxars]

Thanks for the feedback, @ben-eb

The reason why I added the object check was that there is already handling of non-object options in the codebase (postcss-discard-font-face can receive an array, for instance). Be happy to remove it if you think it's not necessary.

Updated PR based on your feedback, awaiting reply regarding object check.

[coveralls]

Coverage remained the same at 100.0% when pulling 1ec5f1489f2af9241da4a179c58e1f12da591be3 on sanity-io:default-plugin-options into 74d8f52939ea41740a7609502c296338cfd1aeb8 on postcss:master.

[ben-eb]

Ah, OK. That makes sense to me. 👍

[ben-eb]

Merged and released on npm in v2.3.0. Thanks again! 🎉