Performance: Make plugin initialization path async

[bryphe]

In Plugin.ts, the plugin initialization path is synchronous:

        if (fs.existsSync(packageJsonPath)) {
            this._oniPluginMetadata = PackageMetadataParser.parseFromString(fs.readFileSync(packageJsonPath, "utf8"))

Making this async (by factoring out to an initialize method that returns a promise, instead of doing two synchronous File I/O calls) will help improve startup performance and responsiveness.