Add a command /timer help that shows the list of all available commands

[dnazarenkov]

Lets add a command /timer help that hows the list of all available commands with short description for each. The result should be like:

Here is a list of possible commands:
start [tasks name]- that starts a timer
stop [tasks name]- that stops timer
status - shows your current status
etc

[xandrck]

It's already implemented in slack, when you type "/timer", slack will show you all available commands with description.