Async FS calls to improve perfomance

[ai]

PostCSS 4.1 brings async support, so we can make postcss-import much faster

[andyjansson]

Added a PR for caching which should also help with performance.

[jednano]

I have a commit here you can cherry-pick to get started. Unfortunately, that code makes the ignore test fail and I can't yet figure out why. I think I've exhausted my willingness to work on this issue, but feel free to take that commit and do what you will with it.

Also, I noticed the use of fs.existsSync in this plugin's code. I want to point out the fact that this will be deprecated along with the asynchronous version, fs.exists. There's also another comment in the docs that states:

In particular, checking if a file exists before opening it is an anti-pattern that leaves you vulnerable to race conditions: another process may remove the file between the calls to fs.exists() and fs.open(). Just open the file and handle the error when it's not there.

fs.exists() will be deprecated.

Maybe the more graceful solution is to just try opening the files and throw any errors that come up.

[MoOx]

6.1.1 has an async option to enable postcss async use, so we might use it to detect async mode and use async fs read. Should not be a big deal from here. Maybe we might use denodeify to transform fs.readFile to a promise.

[MoOx]

I guess we can close, right @TrySound ?