Adding "anystr_lower" to config

[Ronserruya]

Feature Request

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.6.1
            pydantic compiled: True
                 install path: ***
               python version: 3.8.0 (v3.8.0:fa919fdf25, Oct 14 2019, 10:23:27)  [Clang 6.0 (clang-600.0.57)]
                     platform: macOS-10.15.7-x86_64-i386-64bit
     optional deps. installed: ['typing-extensions', 'email-validator']

Just like there is "anystr_strip_whitespace" in the config, which runs .strip() on any string value, I would like to add "anystr_lower" which would run .lower() on any string value.

I just find myself use .lower() on the strings in many validators in places in my code, so I thought it would be useful to add it to the config, I assume that was the use-case of "anystr_strip_whitespace" as well.

If this is fine I'll gladly implement it myself, looks like it's a small change.