ENH: add support for snake case in pandas.Series.str.title
add support for snake case in `pandas.Series.str.title`

https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.str.title.html

so that :
s = pd.Series(['snake', 'CAPITALS', 'IdNumber'])

will be:
```
s
0                 snake
1              capitals
2              id_number
dtype: object
```

This is very much needed when we want to create database columns and they can't have capital letters. lower() isn't good as well because `nextExecutionDateForYearlyBatch` will be impossible to read : `nextexecutiondateforyearlybatch`

Basically I'm looking to do this:
```
import pandas as pd
import janitor
df = pd.DataFrame(...).clean_names()
```
```
Columns before: First Name, Last Name, Employee Status, Subject
Columns after: first_name, last_name, employee_status, subject
```
https://pyjanitor.readthedocs.io/reference/janitor.functions/janitor.clean_names.html
within the `pandas` domain without the need for `janitor` package.