GitLab

Summary
    Make it get a list of installed interpreters, and add the
    relevant local (HOME) site-packages directory to PYTHONPATH.

    Reason
    To easily add all relevant paths, initially, and between upgrades.

Add check for PYTHONUSERBASE

    Summary
    Check for a non-standard install directory, use it if one
    exists, otherwise use ~/.local.

    Reason
    Allow users to specify their own installation directory, without
    affecting functionality.

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