Add functionality to signup

[AntzCode]

After having installed the cli client and started the service, it gives a 403 error because I have not registered an account with the server of the example configuration (https://matrix.org).

Giving the command matrixcli register results in an error:

matrixcli: error: argument subcommand: invalid choice: 'register' (choose from 'send', 'listen', 'rooms', 'unread', 'tail')

It should not be an invalid choice to register via the matrixcli.

Visiting the Matrix.org website tells me that I need to "pick a client and get started".

I should not need to install another application client in order to use the cli client. Furthermore, there is no excuse for having not provided a simple registration form on the website itself.