Rewrite GitHub NPM plugin names to full GitHub tarball links

This allows those plugins to be installed from GitHub even without
having Git installed.

src/plugins.js

@@ -16,11 +16,25 @@ const pluginSources = {
       return new Promise((resolve, reject) => {
         console.log(`Installing ${plugins.length} plugins from NPM...`);
 
+        // Rewrite GitHub npm package names to full GitHub tarball links to avoid
+        // needing to have Git installed to install these plugins.
+
+        // $1 package author, $2 package name, $3 branch (optional)
+        const npmGitHubPattern = /^([a-z0-9_.-]+)\/([a-z0-9_.-]+)(?:#([a-z0-9_.-]+))?$/i;
+        const rewrittenPluginNames = plugins.map(pluginName => {
+          const gitHubPackageParts = pluginName.match(npmGitHubPattern);
+          if (! gitHubPackageParts) {
+            return pluginName;
+          }
+
+          return `https://api.github.com/repos/${gitHubPackageParts[1]}/${gitHubPackageParts[2]}/tarball${gitHubPackageParts[3] ? "/" + gitHubPackageParts[3] : ""}`;
+        });
+
         let stderr = "";
         const npmProcessName = /^win/.test(process.platform) ? "npm.cmd" : "npm";
         const npmProcess = childProcess.spawn(
           npmProcessName,
-          ["install", "--verbose", "--no-save", ...plugins],
+          ["install", "--verbose", "--no-save", ...rewrittenPluginNames],
           { cwd: process.cwd() }
         );
         npmProcess.stderr.on("data", data => { stderr += String(data) });