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

[eamodio]

Related to #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.

[developit]

Sounds like #6 would be ideal here.