Work-in-progress HTTP2 support via "spdy" module

This works, but it looks like the "spdy" module (which, despite its name, also supports http2)
is not compatible with newer nodejs versions and should be avoided:
spdy-http2/node-spdy#333
webpack/webpack-dev-server#1451

Should eventually use nodejs's native http2 module, but it is currently incompatible with express:
expressjs/express#3388

Committing the changes to a branch and setting aside for now.

package.json

@@ -37,6 +37,7 @@
     "morgan": "^1.9.0",
     "nanoid": "^1.1.0",
     "qrcode": "^1.2.2",
+    "spdy": "^3.4.7",
     "superagent": "^3.8.3"
   },
   "devDependencies": {

src/cli.js

@@ -24,6 +24,7 @@ const args = require('meow')(`
       -q, --print-qr           print QR code with the server URL [default: false]
       -Q, --pairing-qr         print QR code with embedded access key [default: false]
       --no-webui               run API server without serving client assets [default: false]
+      --no-http2               disable HTTP/2.0 support [default: enabled when TLS is]
 
       -C, --config-path <path> path to config file [default: ~/.spark-wallet/config]
       -V, --verbose            display debugging information [default: false]

src/transport/tls.js

@@ -1,16 +1,22 @@
 import forge  from 'node-forge'
 import path   from 'path'
-import https  from 'https'
 import http   from 'http'
 import isIp   from 'is-ip'
 import fs     from 'fs'
 import mkdirp from 'mkdirp'
 
 const defaultDir = path.join(require('os').homedir(), '.spark-wallet', 'tls')
 
+// the "spdy" module in fact supports http/2.0 despite its confusing name.
+// we're using it (with spdy support turned off) because the native "http2" module
+// is not currently compatible with Express. https://github.com/expressjs/express/issues/3388
+
+const https  = process.env.NO_HTTP2 ? require('https') : require('spdy')
+    , srvOpt = process.env.NO_HTTP2 ? {} : { protocols: [ 'h2', 'http/1.1', 'http/1.0' ] }
+
 module.exports = (app, name=app.settings.host, dir=defaultDir, leEmail) => {
   const tlsOpt = leEmail ? letsencrypt(name, dir, leEmail) : selfsigned(name, dir)
-      , server = https.createServer(tlsOpt, app)
+      , server = https.createServer( { ...tlsOpt, ...srvOpt }, app)
 
   tlsOpt.cert && app.get('/cert.pem', (req, res) => res.type('pem').send(tlsOpt.cert))
   // @TODO allow downloading letsencrypt's cert