Add "defaults" option for pecl.installed state

This commit adds a "defaults" option, which if True will pipe a newline
to the "pecl install" command, causing default answers to be used to
install extensions which ask questions and thus cannot be installed
unattended.
Docstring tweak

Adds a note to the docstring specifying that the "defaults" feature
won't be available until 0.17.0.
Merge pull request #5882 from terminalmage/issue5701

Add "defaults" option for pecl.installed state