python: add alias to run the proper IPython based on virtualenv

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

plugins/python/README.md

@@ -14,3 +14,4 @@ plugins=(... python)
 | `pyfind`         | Finds .py files recursively in the current directory                            |
 | `pyclean [dirs]` | Deletes byte-code and cache files from a list of directories or the current one |
 | `pygrep <text>`  | Looks for `text` in .py files                                                   |
+| `ipython`        | Runs the appropriate `ipython` version according to the activated virtualenv    |

plugins/python/python.plugin.zsh

@@ -14,3 +14,5 @@ function pyclean() {
 # Grep among .py files
 alias pygrep='grep -r --include="*.py"'
 
+# Run proper IPython regarding current virtualenv (if any)
+alias ipython="python -c 'import IPython; IPython.terminal.ipapp.launch_new_instance()'"