ShellTask Return all lines

[cicdw]

Thanks for contributing to Prefect!

Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

Note that your PR will not be reviewed unless all three boxes are checked.

What does this PR change?

This PR adds a new return_all flag to the ShellTask for optionally returning all lines of stdout; also updates the Changelog following the 0.6.6 release.

Why is this PR important?

There are situations in which this is useful.

[codecov]

Codecov Report

Merging #1598 into master will increase coverage by <.01%.
The diff coverage is 100%.