Add support for Excel date format

## Feature Request
In Excel files, date is represented as an integer that counts how many days have passed from [either January 1st, 1900 (on Windows) or January 1st, 1904 (on Mac)](https://docs.microsoft.com/en-US/office/troubleshoot/excel/1900-and-1904-date-system).

The function [xlrd.xldate.xldate_as_datetime](https://xlrd.readthedocs.io/en/latest/api.html#xlrd.xldate.xldate_as_datetime) from the xlrd library is usually used for parsing this date format in Python. Considering Arrow's goal of providing a common API for date parsing, I believe this would be a useful feature for users who have to wrangle multiple date formats in the same column of an Excel file (because of non-standard data entry).

The only issue is, I'm not sure if this format should be supported as a token to use with arrow.get() and arrow.format() functions (and what that token should be), or if it should be its own function. I'm happy to work on this once we've decided on whether to implement this and how to solve this issue.

No repository file or test is named. Start by comparing the requested behavior with xlrd.xldate.xldate_as_datetime and the existing arrow.get() and arrow.format() entry points, then determine whether this belongs as a token or a separate function. Done means Excel dates using both the 1900 and 1904 systems can be parsed for the intended mixed-format use case.

Written by the indexing model from the issue text.