feat(gui): Add ability to read settings from a config file

This adds the capability to configure settings via a `.etcher.json` file,
either in the user's home directory, or the current working directory.
In the case of the home directory, the config file is `$HOME/.config/etcher/config.json`,
while on Windows `$HOME/.etcher.json` is used.
The defined settings are merged with localStorage settings, and preceding
configuration files.
If both are present, the current working directory takes precedence.

Change-Type: minor
Changelog-Entry: Add support for configuration files

lib/gui/app/models/local-settings.js

@@ -17,6 +17,10 @@
 'use strict'
 
 const Bluebird = require('bluebird')
+const _ = require('lodash')
+const fs = require('fs')
+const path = require('path')
+const os = require('os')
 
 /**
  * @summary Local storage settings key
@@ -25,6 +29,43 @@ const Bluebird = require('bluebird')
  */
 const LOCAL_STORAGE_SETTINGS_KEY = 'etcher-settings'
 
+/**
+ * @summary Local settings filename
+ * @constant
+ * @type {String}
+ */
+const RCFILE = '.etcher.json'
+
+/**
+ * @summary Read a local .etcherrc file
+ * @function
+ * @public
+ *
+ * @param {String} filename - file path
+ * @fulfil {Object} - settings
+ * @returns {Promise}
+ *
+ * @example
+ * readConfigFile('.etcherrc').then((settings) => {
+ *   console.log(settings)
+ * })
+ */
+const readConfigFile = (filename) => {
+  return new Bluebird((resolve, reject) => {
+    fs.readFile(filename, (error, buffer) => {
+      if (error) {
+        if (error.code === 'ENOENT') {
+          resolve({})
+        } else {
+          reject(error)
+        }
+      } else {
+        resolve(JSON.parse(buffer.toString()))
+      }
+    })
+  })
+}
+
 /**
  * @summary Read all local settings
  * @function
@@ -39,9 +80,22 @@ const LOCAL_STORAGE_SETTINGS_KEY = 'etcher-settings'
  * });
  */
 exports.readAll = () => {
+  const homeConfigPath = process.platform === 'win32'
+    ? path.join(os.userInfo().homedir, RCFILE)
+    : path.join(os.userInfo().homedir, '.config', 'etcher', 'config.json')
+  const workdirConfigPath = path.join(process.cwd(), RCFILE)
+  const settings = {}
   return Bluebird.try(() => {
-    return JSON.parse(window.localStorage.getItem(LOCAL_STORAGE_SETTINGS_KEY)) || {}
-  })
+    _.merge(settings, JSON.parse(window.localStorage.getItem(LOCAL_STORAGE_SETTINGS_KEY)))
+  }).return(readConfigFile(homeConfigPath))
+    .then((homeConfig) => {
+      _.merge(settings, homeConfig)
+    })
+    .return(readConfigFile(workdirConfigPath))
+    .then((workdirConfig) => {
+      _.merge(settings, workdirConfig)
+    })
+    .return(settings)
 }
 
 /**