Many packager improvements; Windows signing! (#413)

* Many packager improvements; Windows signing!

* Windows signing works now! (Certs are on an external USB stick that
must be plugged into the build machine during build. We can't do the
same for OS X because certs need to exist in the login Keychain to be
found.)

Fixes #219

* Signing is now optional (so OS X and Windows contributors can run
`npm run package` without errors)

* zip, dmg, and deb arguments are now passed in as e.g. "--package=dmg"

* Print a huge warning when signing is disabled so we're less likely to
ship unsigned binaries to users.

* Make console.logs during packaging consistent and parallel
("creating..." followed by "created.")

* More aggressive signing warnings

* Warn when building OS X app on non-OS X platform (because signing
will never work on non-OS X platforms)
* Warn when building Windows app on non-Windows platform (because
signing doesn't work yet on non-Windows platforms)

package.json

@@ -47,6 +47,7 @@
     "electron-packager": "^6.0.2",
     "electron-winstaller": "feross/windows-installer#build",
     "gh-release": "^2.0.3",
+    "minimist": "^1.2.0",
     "nobin-debian-installer": "^0.0.9",
     "plist": "^1.2.0",
     "standard": "^6.0.5"
@@ -71,7 +72,6 @@
   "scripts": {
     "clean": "node ./bin/clean.js",
     "package": "npm install && npm prune && npm dedupe && node ./bin/package.js",
-    "size": "npm run package -- darwin && du -ch dist/WebTorrent-darwin-x64 | grep total",
     "start": "electron .",
     "test": "standard",
     "update-authors": "./bin/update-authors.sh"