add functions to play with limits + copy application plan

[picsoung]

Meant to answer @lucamaf need to copy an application plan.

This creates:

• a command for limits -> create -> list -> delete

It adds

• the copy command to app-plan

Example of usage:

3scale-cli app-plan copy -s {SERVICE_ID} -a {APPLICATION_PLAN_ID}

[picsoung]

Ping @3scale/support if you have time to test and evaluate this PR :) @lucamaf feel free to try it out too

[picsoung]

Ping @3scale/support to review this PR :)

Thanks