Add pager and nopager commands for redis-cli

[BrotherGao]

As known to all, the pager command is used to display results in mysql(PostgreSQL also has similar commands). The most common usage of pager is to set it to a Unix pager such as less.

I think the pager also applys to redis-cli. It can be very useful to view the result of a command spanning over many lines(e.g., client list, monitor). The status result of Redis maybe more and more in future. I think pager command may be useful in interactive mode for redis-cli.

I hope this PR is useful to redis users.

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❌ 杨东衡
❌ BrotherGao

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