Introduce Deps.merge() method to merge deps
Canonize legacy `make tests` tests results for `bem create -T css` command
Fix Deps.add() to not add duplicated deps to shouldDeps and mustDeps
Fix Deps.parseFull() to clone item, shouldDeps and mustDeps contents
Introduce Deps.parseDepsDecl() method to hide complexity with different deps formats
Refactor `bem decl intersect` and `bem decl subtract` commands to use Deps.parseDepsDecl() method

This also fixes #385
Refactor `bem decl merge` command to use Deps.parseDepsDecl() and Deps.merge() methods
Canonize legacy `make tests` tests results for `bem build -t deps.js`, `bem decl merge` and `bem decl subtract` commands
Canonize `bem make` tests results (deps.js files)