Do not install wasm-pack for webpack projects

[EverlastingBugstopper]

This PR will fix #745 and is blocked by the following action items:

1. Publishing of binary-install to the npm registry
2. Publishing of wasm-pack to the npm registry
3. Merging this PR that allows the wasm-pack-plugin to install wasm-pack via npm by itself without needing Wrangler to handle that installation

[EverlastingBugstopper]

@xtuc - would love feedback on the test fixture I've put in here. I plan on making it so that wasm_pack is only installed if the plugin is included in the webpack config.

This logic may need to live in wranglerjs itself, but we will need to pass the conditional off to wrangler.

[xtuc]

Make sense so far

[xtuc]

https://github.com/cloudflare/wrangler/blob/master/wranglerjs/index.js#L58 fullConfig should contains the computed list of plugins. We'll need to communicate this information to wrangler, via the bundle https://github.com/cloudflare/wrangler/blob/master/wranglerjs/index.js#L92.

[EverlastingBugstopper]

So it seems like the webpack config plugin section looks like this:

 plugins: [ 
  {
    "_ranInitialCompilation": false,
    "crateDirectory": "/Users/averyharnish/Documents/work/_tmp/webpack-rust-worker-template/crate",
    "extraArgs": [],
    "outDir": "pkg",
    "outName": "index",
    "watchDirectories": [
      "/Users/averyharnish/Documents/work/_tmp/webpack-rust-worker-template/crate/src"
    ],
    "watchFiles": [
      "/Users/averyharnish/Documents/work/_tmp/webpack-rust-worker-template/crate/Cargo.toml"
    ],
    "wp": {
      "_events": {},
      "_eventsCount": 0,
      "options": {
        "aggregateTimeout": 200
      },
      "watcherOptions": {},
      "fileWatchers": [],
      "dirWatchers": [],
      "mtimes": {},
      "paused": false,
      "aggregatedChanges": [],
      "aggregatedRemovals": [],
      "aggregateTimeout": 0
    },
    "isDebug": false,
    "error": null
  }
]

unfortunately this doesn't really give me too much identifiable information to work with.

Additionally - it seems like the output from wranglerjs is only created once you actually run the build, and we want to install wasm_pack before the build happens, making this difficult. Would appreciate pointers on this.

[EverlastingBugstopper]

This is currently blocked on rustwasm/wasm-pack#760