Implement `command` function.

[sli]

I've forked this repo and added support for the command function in this form:

command "start" do
  user <- getFrom
  -- and so on

I was originally going to just submit a PR, but I thought it may not be needed since the working implementation is identical to the implementation for hears. That is, the above code is functionally identical to the following:

hears "/start" do
  user <- getFrom
  -- and so on

In short I'm not totally convinced that this library really needs the command function, since it doesn't really add anything new, so I'm posting this issue mostly to find out.

[oliver3]

Hi, thanks for your interest!

Yeah, I was actually wondering the same for the original Telegraf library, but there the decision is made to have the command "alias". I think it is best to follow the Telegraf.js library in this, and add command as well.

Looking forward for your PR :) -- Oliver