Implement --config flag

[novemberborn]

Fixes #1857.

To do:

• Resolve whether the config file needs to be top-level, or whether to resolve glob patterns relative to the directory the config file is in Not accepting globs arguments any more? #2152 (comment)

[novemberborn]

@vladimiry pointed out in #2152 (comment) that users may place their config file in nested directories. We resolve glob patterns and requires relative to the project directory, however. And, since the proposed solution for merging configurations is to use object rest/spread, even if we were to resolve things relative to the config file that doesn't work when you merge with another config file.

So instead, until such time as we have a better solution for this, I've opted to require config files to be in the same directory as the package.json file.

@vladimiry, @sindresorhus thoughts?

[vladimiry]

@novemberborn my opinion won't be relevant here since I'm already using a loader script that resolves the glob pattern and passes the absolute file paths to ava then. But if I were using ava.config.js then I could live with its location to be forced.

[sindresorhus]

So instead, until such time as we have a better solution for this, I've opted to require config files to be in the same directory as the package.json file.

👍

[StarpTech]

LGTM

[StarpTech]

What is blocking this PR?

[novemberborn]

What is blocking this PR?

I wanted to ship it last week, but then I noticed our tests started failing on Node.js 12. I didn't have time to work on this until now.

[novemberborn]

This is now available in v2.2.0.