Add delete/exclude() to DatumInContext

[jbcurtin]

What:
I'm not going to submit this as a pull request, but below is an example of what I'm looking for.
https://github.com/jbcurtin/python-jsonpath-rw/commit/545eecae74c5c574685f2ae78d01b54756f48a82

Why:
Working with postgresql, I'm coming across the need to mask-json data in SQL. I'm going to use this library to write a quick-and-dirty function to do this. By adding the delete/exclude method I can mutate data in a stream using SQL queries.

Cheers!

[kennknowles]

This seems like a good feature request.