Document 'hd' option.
Revamp console system.

Each console now has its own struct to store data, and its thread is
allocated dynamically. The 'struct hydra_console' is defined in
common.h, and can be extended with console-specific fields.

Every function callback gets this console's struct as a parameter,
instead of ChibiOS's BaseStreamSequence output handle. This handle
is included in the struct.

These changes reduce overall code size.
Use t_hydra_console instead of struct hydra_console.
Merge pull request #3 from biot/master

New console system + small doc fix