0.6.0

This release contains a number of improvements and fixes, and adds support for the Konieczny class from libsemigroups.

• Add missing header include in cong.cpp by @james-d-mitchell in #77
• Remove trivial rules by @james-d-mitchell in #79
• Fix string encoding in KnuthBendix by @james-d-mitchell in #82
• Add support for Konieczny from libsemigroups by @james-d-mitchell in #80
• Better __repr__ for KnuthBendix by @james-d-mitchell in #86