Change extra requires to filter on platform

Also created one extra `nativeLib` to replace the orignal two.  Will remove
the others after a couple releases.

README.rst

@@ -22,10 +22,14 @@ issues and fixes would be most appreciated.
 Installation
 ------------
 
-You can download it with pip::
+You can download it with pip:
 
     python -m pip install -U send2trash
 
+To install with pywin32 or pyobjc required specify the extra `nativeLib`:
+
+    python -m pip install -U send2trash[nativeLib]
+
 or you can download the source from http://github.com/arsenetar/send2trash and install it with::
 
     >>> python setup.py install

setup.py

@@ -35,7 +35,14 @@ setup(
     description="Send file to trash natively under Mac OS X, Windows and Linux.",
     long_description=LONG_DESCRIPTION,
     classifiers=CLASSIFIERS,
-    extras_require={"win32": ["pywin32"], "objc": ["pyobjc-framework-Cocoa"]},
+    extras_require={
+        "win32": ['pywin32; sys_platform == "win32"'],
+        "objc": ['pyobjc-framework-Cocoa; sys_platform == "darwin"'],
+        "nativeLib": [
+            'pywin32; sys_platform == "win32"',
+            'pyobjc-framework-Cocoa; sys_platform == "darwin"',
+        ],
+    },
     project_urls={"Bug Reports": "https://github.com/arsenetar/send2trash/issues"},
     entry_points={"console_scripts": ["send2trash=send2trash.__main__:main"]},
 )