Added ephemeral cooldowns - #303

Adds cooldowns to DeluxeMenus.

Cooldowns are ephemeral: they are kept in memory only. They survive /dm reload, but they are gone when the server stops or the plugin unloads (for example a forced /reload).

To start a cooldown, a new action was added:

click_commands:
  - '[ephemeralcooldown] shop_buy 30'

To check a cooldown, a new requirement was added:

click_requirement:
  requirements:
    not_on_cooldown:
      type: '!has ephemeral cooldown'
      id: 'shop_buy'
      deny_commands:
        - '[message] &cWait &f%deluxemenus_ephemeral_cooldown_shop_buy%&cs.'

Some placeholders were added as well:

%deluxemenus_ephemeral_cooldown_<id>% - seconds left
%deluxemenus_ephemeral_cooldown_millis_<id>% - milliseconds left
%deluxemenus_ephemeral_cooldown_formatted_<id>% - 1h 2m 3s, with 0 values not included
%deluxemenus_has_ephemeral_cooldown_<id>% - boolean

• Duration is set in seconds, but accepts s, m, h suffixes and fractions: 30, 0.25, 30s, 5m, 1h.
• New config.yml option max_ephemeral_cooldown_seconds caps how long a cooldown may be set for. Default -1, which means no cap.
• When a cooldown is set above the max limit, it is capped at the max limit and a WARNING is sent in console.

Closes #94