Date utility functions `prevMonth` and `nextMonth` change the source
date when using `src.set[Date|Month|FullYear]()`. This change creates a
copy of the date at entrance of function so we have a pure function.
Made some code refactor and added specs for these functions.
[new] add test config
[new] add breadcrumb test
[change] update package.json
[new] util.js copied from element
[unresolved] see test/unit/index.js @todo
