cmd: Add short name for images and instances cmd

$ capstan i --> $ capstan images
$ capstan I --> $ capstan instances

Signed-off-by: Asias He <asias@cloudius-systems.com>
cmd/run: List all possible options for capstan run -p

Signed-off-by: Asias He <asias@cloudius-systems.com>
cmd/run: List all possible options for capstan run -n

$ capstan run -n bridge

$ capstan run -n nat

Signed-off-by: Asias He <asias@cloudius-systems.com>
cmd/run: Give an example for capstan run -b option

Signed-off-by: Asias He <asias@cloudius-systems.com>
Merge pull request #99 from asias/cmd.improve

A few improvements to capstan cmd