Adding PATCHES keyword. (#558)

* Adding PATCHES keyword.

* Formatting fix.

* Move cpm_add_patches() outside if/else scopes.

* cmake-format: add PATCHES to CPMAddPackage.

* Integration tests for PATCHES command.

* Use get_filename_component() in place of cmake_path() for use with all cmake versions 3.14 and above.

* Added an example and improved comment for cpm_add_patches.

test/integration/templates/using-patch-adder/patches/002-test_patches_command.patch

@@ -0,0 +1,12 @@
+diff --git a/code/adder/adder.cpp b/code/adder/adder.cpp
+index fc6e767..44b1197 100644
+--- a/code/adder/adder.cpp
++++ b/code/adder/adder.cpp
+@@ -1,6 +1,6 @@
+ #include "adder.hpp"
+ 
+-namespace adder
++namespace patched
+ {
+ int add(int a, int b)
+ {