Provide sync support

[StarpTech]

Proposed changes

This feature is needed to provide a sync interface for libraries like:

• https://github.com/giuseppeg/styled-jsx-plugin-postcss/issues/31#issuecomment-588051109 They were forced to use a workaround like https://github.com/abbr/deasync. This PR will use the sync support of cosmiconfig in v4.

Types of changes

What types of changes does your code introduce Put an x in the boxes that apply

• [ ] Bug (non-breaking change which fixes an issue)
• [X] Feature (non-breaking change which adds functionality)
• [ ] Breaking Change (fix or feature which changes existing functionality)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is a reminder of what we are going to look for before merging your code.

• [X] I have read the CONTRIBUTING guide
• [X] Lint and unit tests pass with my changes ()
• [X] I have added tests that prove my fix is effective/works
• [X] I have added necessary documentation (if appropriate)
• [X] Any dependent changes are merged and published in downstream modules

[coveralls]

Coverage remained the same at 100.0% when pulling fc37f1f3bb52990dd75463b1679da21d4b8cf1bb on StarpTech:feature/sync_support into 11f7173d8e84a048ba5d8a9afc8b3a3dfa19d362 on michael-ciniawsky:master.

[StarpTech]

The reason why Node.js 4 is failing is unclear. As far as I can see is that the build issue exists for more than 2 years. It has nothing to do with this feature. Since Node.js 4 is very old I recommend removing it from the matrix (In a new PR).