Simplify support for templates as commonJS files

[tadatuta]

Now to use templates in commonJS format one must:

1. Provide all the methods listed in BEMXJST.prototype.locals in exactly the same order to exported function
2. Make first empty call of compile as first template is always considered to be string

E.g.

// b1.bemtree.js
module.exports = function(
  match,
  block,
  elem,
  mod,
  elemMod,
  oninit,
  xjstOptions,
  …
) {
    block('b1').content()('i am b1');
};

// server.node.js
const bemtree = require('bem-xjst').bemtree;

const b1 = require('./blocks/b1.bemtree');

const tmpl = bemtree
    .compile() // this is the second point, expected to be not necessary 
    .compile(b1);

const bemjson = tmpl.apply({ block: 'b1' });

Let's support possibility to call template function with just one object argument having all needed methods so it could be used like this:

module.exports = function({block}) {
    block('b1').content()('i am b1');
};

I believe it should be possible to support such behavior as an option to make it backward compatible .