new tech to freeze files in html
Freezes script@src, img@src, link@href
relative paths in require
unit tests
unit tests
unit tests
Add flexibility to freeze

So I can extend tech with my custom extensions in this way

my-tech.js
```js
isFreezableUrl: function(url) {
  return this.__base(url) || myRe.test(url)
}
```
remove process.env.BORSCHIK_FREEZABLE_EXTS
Merge pull request #81 from doochik/freeze-html

New tech to freeze files in html