Prefer usage of "python -m pip" instead of "pip" (#1059)

* Prefer usage of "python -m pip" instead of "pip"

It will prevent issues when users think that they are using
the correct `pip` version. It can refers to the one from the OS
Python installation, or even worse to a Python 2 installation.
Let's be clear on how to install stuff.

Also used short version of `pip` arguments, because we are all lazy :)

* Apply suggestions from code review

CHANGELOG.rst

@@ -172,7 +172,7 @@ This project adheres to `Semantic Versioning <https://semver.org/>`_.
 * Added fish shell completion (located in ``extras/httpie-completion.fish``
   in the GitHub repo).
 * Updated ``requests`` to 2.10.0 so that SOCKS support can be added via
-  ``pip install requests[socks]``.
+  ``python -m pip install requests[socks]``.
 * Changed the default JSON ``Accept`` header from ``application/json``
   to ``application/json, */*``.
 * Changed the pre-processing of request HTTP headers so that any leading

CONTRIBUTING.rst

@@ -193,7 +193,7 @@ Install HTTPie in editable mode with all the dependencies:
 
 .. code-block:: powershell
 
-    pip install --upgrade -e . -r requirements-dev.txt
+    python -m pip install --upgrade --editable . -r requirements-dev.txt
 
 You should now see ``(httpie)`` next to your shell prompt, and
 the ``http`` command should point to your development copy:

README.rst

@@ -175,7 +175,7 @@ Otherwise with ``pip``:
 
 .. code-block:: bash
 
-    $ pip install --upgrade https://github.com/httpie/httpie/archive/master.tar.gz
+    $ python -m pip install --upgrade https://github.com/httpie/httpie/archive/master.tar.gz
 
 
 Verify that now we have the