wea.table_filter: add API function

Really, Lua should have this already. At this rate, I'll be implementing 
the entire Javascript standard library... :P

worldeditadditions/utils/tables.lua

@@ -69,7 +69,7 @@ end
 --- Executes the given function on every item in the given table.
 -- Ignores return values that are nil and doesn't insert them into the table.
 -- @param	tbl		table		The table to operate on.
--- @param	func	function	The function to execute on every item in the table.
+-- @param	func	function<any>:any|nil	The function to execute on every item in the table.
 -- @returns	table	A new table containing the return values of the function.
 function worldeditadditions.table_map(tbl, func)
 	local result = {}
@@ -79,6 +79,22 @@ function worldeditadditions.table_map(tbl, func)
 	end
 end
 
+--- Filters the items in the given table using the given function.
+-- The function is executed for each item in the list. If it returns true, the
+-- item is kept. If it returns false, the item is discarded.
+-- @param	tbl		table		The table of values to filter.
+-- @param	func	function<any>:bool	The filter function to execute - should return a boolean value indicating whether the item provided as the first argument should be kept
+-- @returns	table	A new table containing the values that the given function returned true for.
+function worldeditadditions.table_filter(tbl, func)
+	local result = {}
+	for i,value in ipairs(tbl) do
+		if func(value) then
+			table.insert(result, value)
+		end
+	end
+	return result
+end
+
 --- Builds a new table with the elements of the given table appearing at most once.
 -- @param	tbl		table	The table of values to make unique.
 -- @returns	table	A new table containing the values of the given table appearing at most once.