Add add-proc-name command

[GaetanLepage]

This allows to add a process and to give it a name directly.

Example:

mprocs --server 127.0.0.1:4050 --ctl '{c: add-proc-name, name: "main-server", cmd: "./run-server.sh --port 1234"}'

Warning, my rust skills are very limited and this is no more than an honest and naive attempt at implementing this feature.

Alternatively, we could simply add an optional name field to the existing add-proc command.
As I haven't found any example of optional fields in the existing commands, I have went with adding a brand new command.
However, I would feel like this would be a cleaner solution.

Also, this implementation adds duplicated code in app.rs which is shared with the AddProc event.
I wasn't sure how and where to factorize this snippet.