Adds support for clickable links

Those changes adds in support for links that can be clicked to send a command back to the server.
The links are sent like this:

    msg("Exits: {lcgo south{ltsouth{le")
(lc starts for link command, lt for link text and le for link end)
This makes the links somewhat consistent with the existing Evennia formating codes. But feel free to change those to whatever :)

On the protocol side: this is implemented using MXP for telnet clients that support it and just plain html/javascript for the webclient. Other protocols will just see the link text without being able to click on them.

A working implementation of this has been added in the menu system so a menu can be used by just clicking on menu items.