src/ln: document filters, let them reject lines

README.md

@@ -55,3 +55,22 @@ local lgr = ln.Logger:new()
 lgr:log {foo = "bar"}
 -- time="2019-12-25T13:27:32" foo=bar
 ```
+
+Or custom filters
+
+```lua
+local lgr = ln.Logger:new {
+  filters = {
+    function(message)
+      if string.find(message, "debug=true") and os.getenv("DEBUG") ~= "YES" then
+        return false
+      end
+    end,
+    print,
+  }
+}
+
+lgr:log {debug = "true", line = "PRIVMSG #foo :bar", dir = "out"}
+```
+
+This will make the log line only show up when `DEBUG` is set to `"YES"`.

spec/ln_spec.lua

@@ -91,5 +91,16 @@ describe("ln", function()
       lgr:log {foo = "bar"}
       assert.truthy(string.find(msg, 'foo=bar'))
     end)
+    
+    it("ends if a filter returns non-nil", function()
+      msg = nil
+      lgr.filters = {
+        function(message) return false end,
+        filter_func,
+      }
+      
+      lgr:log {foo = "bar"}
+      assert.falsy(msg)
+    end)
   end)
 end)

src/ln.lua

@@ -65,7 +65,9 @@ function Logger:log(...)
 
   local message = self.formatter:format(result)
   for _, v in pairs(self.filters) do
-    v(message)
+    if v(message) ~= nil then
+      return
+    end
   end
 end