Support configurable settings

[hackmod]

• separate configurable variables to the config.default.json
• read local config.json to override default variables

There are several MyEtherWallet based well-known clone sites exist like as

• https://pyrus.ubiqsmart.com/
• https://wallet.pirl.io/
• https://w.akroma.io/
• https://wallet.ether1.org/
• https://myegemwallet.com/
  and so on.

some wallets are updated frequently based on the latest MyEtherWallet but not all of them frequently updated.

This PR makes clone sites more likely maintainable.

Done

• extract configurable variables to config.default.json
• customLess option : append selected customLess file
• configurable network list using gulp prep --networks eth_ethscan,pirl command

config.default.json

{
  "selected": "mew",
  "mew": {
    "custom": false,
    "showDontGetPhished": true,
    "showPrefooterWarning": true,
    "showMewAdSites": true,
    "showDonation": true,
    "showTranslators": true,
    "showMewSites": true,
    "showGithub": true,
    "showExtensions": true,
    "googleSiteVerification": "IpChQ00NYUQuNs_7Xs6xlnSdzalOlTUYbBsr8f7OpvM",
    "legalname": "MyEtherWallet Inc",
    "sitename": "MyEtherWallet",
    "sitetitle": "Your Key to Ethereum",
    "sitehost": "MyEtherWallet.com",
    "siteurl": "https://www.myetherwallet.com",
    "email": "support@myetherwallet.com",
    "description": "MyEtherWallet (MEW) is a free, open-source, client-side interface for generating Ethereum wallets & more. Interact with the Ethereum blockchain easily & securely.",
    "descriptionFixed": "MyEtherWallet.com is a free, open-source, client-side interface for generating Ethereum wallets & more. Interact with the Ethereum blockchain easily & securely.",
    "shortDescription": "Free, open-source, client-side Ethereum wallet. Enabling you to interact with the blockchain easily & securely.",
    "appTitle": "MyEtherWallet · Your Key to Ethereum",
    "themecolor": "#1d6986",
    "maskcolor": "#2f99b0",
    "twitterid": "@MyEtherWallet",
    "sitelogo": "logo-myetherwallet.svg",
    "sitelogocx": "logo-myetherwalletcx.svg",
    "poweredby": "logo-myetherwallet.svg",
    "logourl" : "https://www.myetherwallet.com/images/myetherwallet-logo.png",
    "copyright": "© 2018 MyEtherWallet, Inc",
    "instagram": "https://instagram.com/myetherwallet",
    "reddit": "https://www.reddit.com/r/MyEtherWallet/",
    "twitter": "https://twitter.com/myetherwallet",
    "facebook": "https://www.facebook.com/MyEtherWallet/",
    "medium": "https://medium.com/@myetherwallet",
    "linkedin": "https://www.linkedin.com/company/myetherwallet",
    "support": "mailto:support@myetherwallet.com",
    "github": "https://github.com/MyEtherWallet"
 }