Update craft-plain-ics to be a Craft 3 plugin

[jcdarwin]

This converts craft-plain-ics to be a Craft 3 plugin.

As such, you might like to either:

• create a craft 2 specific branch from the existing (craft 2) master branch, and then merge this to master (i.e. master becomes craft 3)
• keep the (craft 2) master branch as is, and merge this to a new craft 3 branch

Note that:

• I've tried to keep all attribution etc as it was previously
• we remove the vendors folder (which reduces the size of this project quite a bit), as we are using composer in Craft 3, and eluceo/ical and plainlanguage/craft-plain-ics end up as siblings in craft/vendor

Please let me know if you've any feedback or suggestions.

[jcdarwin]

Anyone home?