Add `autoenv_new()` for quick creation of .env

Addresses Issue #42.

Add simple new feature: a function `autoenv_new()` that checks for the existence of `.env` (or `$1`), and then, if it does not exist, creates `.env` with a one-liner to activate/create a virtualenv named after the present working directory (via `use_env` (see [Cookbook](https://github.com/hangtwenty/autoenv/wiki/Cookbook)).