Rework public API, clean up purpose of the module

[qfox]

@tadatuta said this module should ~store~ stringify entities since it's called bemjson2decl and decl is a file ...bemdecl.js (we've made an issue for that: #13)

I'm see we need to make public API clear, write docs and work on consistency between bem-sdk modules.

What we have:

• convert(bemjson): BemEntityName[]
• stringify(bemjson): String - returns formatted string with decl content, like bemDecl.stringify do, not sure we need a clone.

What I expect:

• export function instead of object that will do the same as convert do but return BemCell[]: function(bemjson): BemCell[]
• prob drop stringify(bemjson) since we have bemDecl, or, as @tadatuta said, just alias it to bemDecl.stringify and add opts with format and exportType params.

Something ilke:

module.exports = bemjsonToDecl;

bemjsonToDecl.convert = bemjsonToDecl; // To not make back-incomp changes
bemjsonToDecl.stringify = (bemjson, opts) => bemDecl.stringify(bemjsonToDecl(bemjson), opts);

function bemjsonToDecl(bemjson) {
    ...
}

[Yeti-or]

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