Add shell command (#201)

* Add shell command

* Make shell command Python2-compatible

* Add provisional Windows support; handle unset SHELL variable

* Reformatted with black

* Handle already activated virtualenvs

* Add comments to shell command code

* Remove splat operator to support Python 2

* Use "1" for POETRY_ACTIVE instead of virtualenv path

* Handle non-existing virtualenv when virtualenvs.create is false

* Handle unexisting virtualenvs in a broader way

* Check if POETRY_ACTIVE is specifically set to a true value

* Use utils.shell to get path to running shell