Command overhaul

[osa1]

As discussed in #61,

Current command implementation has some issues and/or could be improved.

• [ ] No command auto completion
• [x] No way to get help about a command from within the app (or a /help command that lists all commands)
• [ ] Poor error checking and error messages

I'm hoping to fix these. As usual contributions are welcome.

Other commands-related issues: #63 (/join doesn't support password-protected channels), #59 (/connect doesn't allow connecting with TLS).

[meain]

Also maybe add a way to set alias for commands in config file

[osa1]

/help command implemented in ec00007.