add prepare script to allow yarn/npm load from git

[sneak]

By adding a prepare stanza to the package.json scripts, this allows a consumer to specify the dependency by git URL and have it transpile properly when npm install is run in the consumer.

{
  "name": "steem-program",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT",
  "dependencies": {
    "@steemit/steem-js": "https://github.com/steemit/steem-js"
  }
}

[sneak]

More info available at npm/npm#3055

[sneak]

lol CI fails because the test is using the network and the test account is bandwidth limited:

  1) steem.broadcast: comment with options works:
     RPCError: plugin exception:Account: guest123 has 966821953 RC, needs 1443846168 RC. Please wait to transact, or power up STEEM.rethrow
      at RPCError.Error (native)
      at new RPCError (src/api/transports/http.js:8:25)
      at src/api/transports/http.js:36:13
      at process._tickCallback (internal/process/next_tick.js:109:7)