Restore older SimpleOptions parsing behavior for values with spaces

Summary:
Previously, you could do `name=blah blah blah`. After the recent update, we require spaces to be quoted.

Instead, permit unescaped spaces in values but not in keys. This fixes some older memes from pre-escape times.

Test Plan: Ran unit tests.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D5161