feat: add support for embark.config.js

This commit introduces support for using `embark.config.js` to calculate the
embark configuration object that is otherwise provided via `embark.json`.

If an `embark.config.js` file is present, it will be used over the
`embark.json` file.  The `embark.config.js` module needs to export either an
object or a function that can be asynchronous and has to return or resolve with
an embark configuration object:

```js
// embark.config.js

module.exports = async function () {
  let config = ...; // do lazy calculation of `embarkConfig`;
  return config;
}
```

packages/core/console/src/index.ts

@@ -169,7 +169,7 @@ export default class Console {
       return this.ipc.request("console:executeCmd", cmd, callback);
     }
 
-    if (cmd.indexOf("profile") === 0 && warnIfPackageNotDefinedLocally("embark-profiler", this.embark.logger.warn) !== true) {
+    if (cmd.indexOf("profile") === 0 && warnIfPackageNotDefinedLocally("embark-profiler", this.embark.logger.warn, this.embark.config.embarkConfig) !== true) {
       return callback(null, "please install embark-profiler plugin");
     }
     if (!(cmd.split(" ")[0] === "history" || cmd === __("history"))) {

packages/core/engine/src/index.ts

@@ -121,8 +121,8 @@ export class Engine {
   }
 
   async generateEmbarkJSON() {
-    if (fs.existsSync('embark.json')) {
-      throw new Error(__('embark.json already there. Will not overwrite'));
+    if (fs.existsSync('embark.json') || fs.existsSync('embark.config.js')) {
+      throw new Error(__('embark.json or embark.config.js already exist. Will not overwrite'));
     }
     return fs.writeFile('embark.json', JSON.stringify(constants.defaultEmbarkConfig, null, 2));
   }

packages/core/utils/src/index.ts

@@ -313,7 +313,7 @@ export function isEs6Module(module) {
   return (typeof module === 'function' && isConstructor(module)) || (typeof module === 'object' && typeof module.default === 'function' && module.__esModule);
 }
 
-export function warnIfPackageNotDefinedLocally(packageName, warnFunc) {
+export function warnIfPackageNotDefinedLocally(packageName, warnFunc, embarkConfig) {
   const packageIsResolvable = findUp.sync("node_modules/" + packageName, {cwd: dappPath()});
   if (!packageIsResolvable) {
     return warnFunc("== WARNING: "  + packageName + " could not be resolved; ensure it is defined in your dapp's package.json dependencies and then run npm or yarn install; in future versions of embark this package should be a local dependency and configured as a plugin");
@@ -325,7 +325,6 @@ export function warnIfPackageNotDefinedLocally(packageName, warnFunc) {
     return warnFunc("== WARNING: it seems "  + packageName + " is not defined in your dapp's package.json dependencies; In future versions of embark this package should be a local dependency and configured as a plugin");
   }
 
-  const embarkConfig = fs.readJSONSync(dappPath("embark.json"));
   if (!embarkConfig.plugins[packageName]) {
     return warnFunc(
       __("== WARNING: it seems %s is not defined in your Dapp's embark.json plugins;\nIn future versions of Embark, this package should be a local dependency and configured as a plugin", packageName)