Add CI_User_agent::parse() to allow parsing a custom user-agent string

Based on PR #970
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index 67a2685..cbc1c66 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -369,7 +369,11 @@
       -  Added the ability to use a proxy.
       -  Added Basic HTTP authentication support.
 
-   -  :doc:`User Agent Library <libraries/user_agent>` will now check if robots are pretending to be mobile clients (helps with e.g. Google indexing mobile website versions).
+   -  :doc:`User Agent Library <libraries/user_agent>` changes include:
+
+      - Added check to detect if robots are pretending to be mobile clients (helps with e.g. Google indexing mobile website versions).
+      - Added method ``parse()`` to allow parsing a custom user-agent string, different from the current visitor's.
+
    -  Added support for setting :doc:`Table <libraries/table>` class defaults in a config file.
    -  :doc:`Zip Library <libraries/zip>` method ``read_file()`` can now also alter the original file path/name while adding files to an archive.