Make bemjson and bemdecl optional

[Yeti-or]

Case: I want abstraction over fs for bem bundles and this module is suitable, but i want to collect bemjson or decls later

[Yeti-or]

possible solution: {bemjson: undefined, decl: null} plus add setters

asserts:

• get bemjson -> assert(decl null)
• set bemjson -> assert(bemjson !undefined)
• get decl -> assert(decl null)
• set decl -> assert(decl !null)

cc @zxqfox