variables for development environment

[jifeon]

Now bnsf looks at BEMHTML_ENV=development, BEMTREE_ENV = development env variables to recognize development environment, but there is NODE_ENV variable which used by many other libraries. Maybe we should look at this if BEMHTML_ENV and BEMTREE_ENV are not set?

[apsavin]

Actually bnsf doesn't looks at those env vars, but yes, we can do this enhancement.