Add onRune parameter, utf8 package in plugins.md

[niten94]

The bufpane where a rune is entered is passed when onRune, preRune are called and unicode/utf8 can be used in plugins, but the parameter and package were not written in plugins.md so they have been added in the file in this pull request.

[niten94]

There are only packages that are in the Go standard library in the list where unicode/utf8 was added, but humanize is not in the standard library so I do not think it can be added there. It is written that humanize can be used in plugins under the list so I do not think something has to be added about the package.