Change API of bem-environ to one more straightforward

- make environ exported object a function to set global root
- make lib/arch export a function that accept nodes registry and environ object as its arguments
- add environ.extendMake() function to apply nodes extensions
- reflect changes in a readme

Current usage is:

```js
// .bem/make.js

// get environ object and init global root
var environ = require('bem-environ')(__dirname);

// extend `bem make` flow with `bem-environ` nodes
environ.extendMake(MAKE);
```