Compatibility with clean-webpack-plugin (or just any plugin to clean the build dir before the build

Related to https://github.com/GoogleChromeLabs/size-plugin/issues/6 but different in that I only want the data to be cached right at the start, so that a clean build can still be performed.

Also I'm running into issues when trying to use both this plug-in and `clean-webpack-plugin`:

```
(node:29504) UnhandledPromiseRejectionWarning: Error: ENOENT: no such file or directory, open 'C:\Users\Eric\code\vscode-gitlens\dist\extension.js'
(node:29504) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:29504) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
Warning: The 'no-floating-promises' rule requires type information.
(node:29504) PromiseRejectionHandledWarning: Promise rejection was handled asynchronously (rejection id: 1)
{ Error: ENOENT: no such file or directory, open 'C:\Users\Eric\code\vscode-gitlens\dist\extension.js'
errno: -4058,
code: 'ENOENT',
syscall: 'open',
path: 'C:\\Users\\Eric\\code\\vscode-gitlens\\dist\\extension.js' }
```

In my webpack config, I was adding `new SizePlugin()` as the first plugin.