Running webpack --watch triggers plugin twice on start

Running Webpack in watch mode kicks off the Plugin twice.

I'm not 100% why, but I think it has something to do with the fact that ESDoc itself seems to drop a file into your source directory, temporarily, when it runs its own plugins: `.external-ecmascript.js`.

Additionally, it could be due to the fact that this project is creating a tmp file to house the combined configuration options from the given configuration file and any options you pass to ESDoc from your webpack configuration.

Or it could be that when the docs are emitted by the child process, the watch proceses sees the change and decides to go again.

Whatever the case, it's annoying.