Current state: a user invokes `ipython` and is provided with the IPython
instance regarding the `$PATH`.

Proposed state: a user invokes `ipython` (which is a new alias in the
*python plugin*) and is provided with the proper IPython instance regarding
the currently activated virtualenv.

Example: the user's default Python is 2.7 with installed IPython 2.7. User
activates Python 3.5 virtualenv where he installs IPython 3.5. After
activating the environment, one expects `ipython` to run the version 3.5,
which does not happen by default. Instead, IPython 2.7 is used, which in
counter-intuitive and often causes problem.

Closes #5797