Explore the possibility to add a command to copy evaluation result

[st0012]

It could be very helpful to have a command to copy IRB's output without manually selecting a (potentially huge) chunk of text. For example:

copy User.all.to_a # copies and echos all users
copy User.all.to_a; # copies and doesn't echo all users
copy _ # copies the previous output value

Things to consider:

• We cannot rely on any non-default Ruby gems
• We may use system commands if they're widely available, at least among UNIX-based systems