Adding a custom line delimiter to iter_lines()

Initial description is in Issue #2292 however, I shall elaborate.

It is often necessary to iterate over something other than conventional newlines, where such characters are a part of the data, for example.  This is a small functionality requiring a proportionally small change in the code.  Wrapping `iter_content()` in a custom iterator outside of the Requests library requires significantly more code, which is why I'm proposing this change.  The effect on the performance is negligible.