implement /mtimer set chat command

Issue https://github.com/dsohler/mtimer/issues/1

system/chat_commands.lua

@@ -13,6 +13,43 @@ local colorize = function (color, string)
 end
 
 
+-- Validate a new value
+--
+-- 1. Check if setting is valid at all
+-- 2. If `show` has to be set, validate for 'true' or 'false' string
+-- 3. Validate against a simple regex
+-- 4. If all of the above was not `nil`, return `true`
+--
+-- @param setting   The setting the value is for
+-- @param new_value The new value that has to be validated
+-- @return bool     If the new value for the setting is valid or not
+local validate = function (setting, new_value)
+    local valid_values = {
+        runtime = '^[^%c]+$',
+        offset = '^%-?[0-9]+,%-?[0-9]+$',
+        ingame_time = '^[^%c]+$',
+        current_time = '^[^%c]+$',
+        font_color = '^[0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f]$',
+        start = '^[^%c]+$',
+        show = '^$', -- unmatchable, see below
+        locale = '^$', -- currently unsupported and not settable by user
+        position = '^%-?[0-9]+,%-?[0-9]+$',
+        format = '^[^%c]+$',
+        alignment = '^%-?[0-9]+,%-?[0-9]+$'
+    }
+
+    if valid_values[setting] == nil then return false end
+
+    -- Check if `new_value` for setting `show` is either 'true' or 'false' and
+    -- return true in both cases (because either 'true' or 'false' are valid).
+    if setting == 'show' and new_value == 'true' then return true end
+    if setting == 'show' and new_value == 'false' then return true end
+
+    if new_value:match(valid_values[setting]) == nil then return false end
+    return true
+end
+
+
 -- Show help to the player
 --
 -- @param player The player to show the help to
@@ -67,6 +104,36 @@ local show_configuration = function (player)
 end
 
 
+local set = function (player, payload)
+    local name = player:get_player_name()
+    local setting = payload:match('[%a_]+')
+    local new_value = payload:gsub('^[%a_]+ ', '')
+    local old_value = player:get_attribute('mtimer:'..setting)
+    local status = ''
+
+    if old_value == nil then
+        minetest.chat_send_player(name, colorize('ef2929', 'Unknown setting!'))
+        show_configuration(player)
+        return
+    end
+
+    if validate(setting, new_value) == false then
+        minetest.chat_send_player(name, colorize('ef2929', 'Invalid value!'))
+        show_configuration(player)
+        return
+    end
+
+    status = status..colorize('729fcf', '[MTimer]')..' '
+    status = status..'Setting '..colorize('73d216', setting)..' to '
+    status = status..colorize('75507b', new_value)
+
+    print('[MTimer] '..name..' sets `'..setting..'` to `'..new_value..'`')
+    minetest.chat_send_player(name, status)
+    player:set_attribute('mtimer:'..setting, new_value)
+    mtimer_update(player)
+end
+
+
 -- Register the chat command that will be used to interact with MTimer.
 --
 -- The chat command has three actions: