xticks beyond those in DataFrame

I found myself looking at trends and wanting to pseudo forecast them into the future by simply mouse over-ing at some future date. However, since that date is not in the DataFrame, there is neither tick marks nor the x,y display that is shown for all of the data.

I hacked together a solution by modifying the `IntegerIndexDateTimeFormatter`, which solves both tick marks and the x,y display.

I thought I'd post here in case it is useful to anyone else. I'm not sure if I know mplfinance well enough to know if it breaks other things or if it is worth creating a pull request.

https://github.com/mikessut/mplfinance/tree/dates_bynd_data

Start with the IntegerIndexDateTimeFormatter change in the linked dates_bynd_data branch and reproduce the missing tick marks and x,y display for dates beyond the DataFrame. Compare the behavior with existing plots and check whether the change preserves current in-range date handling; done means future dates can be inspected without breaking existing formatting.

Written by the indexing model from the issue text.