Allow use with HarfBuzz renderer.

[eclecticfluff]

Up to and including version 0.333, selnolig has problems when used with the Harfbuzz engine set with fontspec. The problems range from inconvenient like here to catastrophic like I reported with junicode psb1558/Junicode-font#310. At the very least, selnolig should emit a warning when used with Harfbuzz until a solution can be found so that users are aware of the risks.

[micoloretan]

Thank you for contacting me. I will take your suggestions into account when I have a chance to apply a maintenance update. Best, Mico