fixed Pry::Editor#invoke_editor

Pry::Editor doesn't have a class method called invoke_editor. Fixed to call on an initialized Pry::Editor instance
Merge pull request #1298 from dsisnero/patch-1

fixed Pry::Editor#invoke_editor
add 'gem-readme' command.
Merge pull request #1321 from rpag/gem-readme

add 'gem-readme' command.
group gem-readme with other gem-* commands
add 'gem-search' command