Add commands to control the timeout

[lmoureaux]

Add three new timeout-related server commands:

• timeoutshow displays the current timeout
• timeoutadd adds time to the current turn
• timeoutset sets the remaining time for the current turn

They are named timeoutsomething for three reasons:

• timeoutincrease already exists
• it's easier to find them if they're together in /help commands
• sub-commands like "timeout show" are not feasible with the current
  implementation

This is heavily inspired from an earlier patch used by LT with 2.6,
41cc12b401fe4a9db8d22b78039561710fff59e3. The syntax is however more flexible,
allowing for hours or minutes to be passed directly without doing any
calculation.

Closes #1151.

[psampathkumar]

Works! It interprets negative values too, to reduce timeout, (we can even do stuff like 1:-1 to increase by 59 secs). I think this needs to be documented in the help.

You can either create a issue to document server commands in RTD in general and then merge it, or add some help here for the ingame version.

[lmoureaux]

New commit:

Both timeoutset and timeoutadd accept negative value for each part of their time argument (hours, minutes, seconds). I think using this feature shouln't be encouraged as it makes the time less intuitive, but I see no reason to add more code to prevent it.

Still, documenting that timeoutadd can be used to reduce the timeout is important, so mention that it accepts negative values. Creative users may discover that 1:-10 is 50 seconds by themselves.