[ENH] Add ability to convert CamelCase to snake_case by default in clean_names

[ericmjl]

Brief Description

clean_names() right now does not convert names from camel case to snake case. I think it should, and in the spirit of being opinionated, it should be enabled by default.

Example API

The current function signature is:

def clean_names(df, strip_underscores, case_type: str = 'lower', ...):

I would propose changing case_type to default to snake, rather than lower.

def clean_names(df, strip_underscores, case_type: str = 'snake', ...):

Notes

Some Googling around reveals the following implementations for CamelCase to snake_case conversion:

• A GitHub gist
• StackOverflow post

If the implementation is copied over from these two sources, I would ensure that the original code is properly referenced with at least a URL pointing to the original in the docstring.