Docs: add shell quoting to install commands

In docs install examples, show double quotes around
package specifiers that include square brackets, to
prevent them from being interpreted as shell globs.
(Helps with installation on Windows and zsh, e.g.)

Closes #188

README.rst

@@ -77,7 +77,7 @@ or SparkPost or any other supported ESP where you see "mailgun":
 
    .. code-block:: console
 
-        $ pip install django-anymail[mailgun]
+        $ pip install "django-anymail[mailgun]"
 
    (The `[mailgun]` part installs any additional packages needed for that ESP.
    Mailgun doesn't have any, but some other ESPs do.)

docs/esps/amazon_ses.rst

@@ -27,7 +27,7 @@ backend. Either include the "amazon_ses" option when you install Anymail:
 
     .. code-block:: console
 
-        $ pip install django-anymail[amazon_ses]
+        $ pip install "django-anymail[amazon_ses]"
 
 or separately run `pip install boto3`.
 

docs/esps/sparkpost.rst

@@ -17,7 +17,7 @@ backend. Either include the "sparkpost" option when you install Anymail:
 
     .. code-block:: console
 
-        $ pip install django-anymail[sparkpost]
+        $ pip install "django-anymail[sparkpost]"
 
 or separately run `pip install sparkpost`.
 

docs/installation.rst

@@ -12,7 +12,7 @@ To use Anymail in your Django project:
 
     .. code-block:: console
 
-        $ pip install django-anymail[sendgrid,sparkpost]
+        $ pip install "django-anymail[sendgrid,sparkpost]"
 
    The `[sendgrid,sparkpost]` part of that command tells pip you also
    want to install additional packages required for those ESPs.