Make the plugin resolve paths synchronously, add extensions options and a custom regexp.

[cipix2000]

Make the plugin resolve paths synchronously, add an extensions option

and a custom regexp.

Also new APIs are added:

• getRegex - allows a configurable way to name the folders ( for example @fallback instead of #fallback#)
• pathMatchesPrefix
• resolveComponentPath

We can use the last ones in custom resolvers, for example for a custom css-import resolve function we can have:

const resolveFile = path =>
    fallbackDirectoryResolverPlugin().pathMatchesPrefix(path) ? fDRP.resolveComponentPath(path) : path;

The reason for making this plugin synchronous is that the eslint webpack imports plugins doesn't work with resolver plugins that return promises.