Add options for `repl` to disable extra commands
Add options for `repl` to disable extra commands

For my use case I do not want to allow users to run external commands
(as I need to run my CLI's REPL as a priviliged user but only want to
allow access to commands within my Click CLI), and I do not want to
handle the existing internal commands (as I am handling displaying help
and quitting myself). This commit adds parameters to `repl` which will
support both of these situations, while still retaining the existing
default behaviour in both cases.

click-contrib/click-repl