(new): add log level helpers and HTTP method levels

- move LogLevel and predefined levels into levels.go
- add level-specific constructors for info/warn/error/fatal/debug
- add predefined HTTP method log levels and LogLevelForMethod
- unify Logger Print/Println internals and add Printf
- improve GoDoc coverage for exported APIs

colors.go

@@ -5,12 +5,25 @@ import (
 	"strings"
 )
 
+// FgColor is a basic ANSI foreground color code.
 type FgColor uint8
+
+// BgColor is a basic ANSI background color code.
 type BgColor uint8
+
+// FgColor256 is an ANSI 256-color foreground value.
 type FgColor256 uint8
+
+// BgColor256 is an ANSI 256-color background value.
 type BgColor256 uint8
+
+// BgColorRGB is an ANSI truecolor background value encoded as RGB bytes.
 type BgColorRGB []uint8
+
+// FgColorRGB is an ANSI truecolor foreground value encoded as RGB bytes.
 type FgColorRGB []uint8
+
+// Attribute is an ANSI text attribute code.
 type Attribute uint8
 
 // NewBgColorRGB builds an RGB background color value.
@@ -23,6 +36,7 @@ func NewFgColorRGB(r, g, b uint8) FgColorRGB {
 	return FgColorRGB{r, g, b}
 }
 
+// Text attribute codes.
 const (
 	Reset Attribute = iota
 	Bold
@@ -35,6 +49,8 @@ const (
 	Conceal
 	CrossedOut
 )
+
+// Additional text attribute codes.
 const (
 	DoubleUnderline Attribute = iota + 21 // Can be disable bold on some terminals
 	DisableBold
@@ -45,6 +61,8 @@ const (
 	DisableConceal
 	DisableCrossedOut
 )
+
+// Basic foreground color codes.
 const (
 	FgNone          = 0
 	FgBlack FgColor = iota + 30
@@ -57,6 +75,8 @@ const (
 	FgWhite
 	FgDefault FgColor = 39
 )
+
+// Basic background color codes.
 const (
 	BgNone          = 0
 	BgBlack BgColor = iota + 40
@@ -69,6 +89,8 @@ const (
 	BgWhite
 	BgDefault BgColor = 49
 )
+
+// High-intensity foreground color codes.
 const (
 	FgHiBlack FgColor = iota + 90
 	FgHiRed
@@ -79,6 +101,8 @@ const (
 	FgHiCyan
 	FgHiWhite
 )
+
+// High-intensity background color codes.
 const (
 	BgHiBlack BgColor = iota + 100
 	BgHiRed
@@ -90,7 +114,7 @@ const (
 	BgHiWhite
 )
 
-// This attributes rarely used and not supported by all terminals.
+// Rarely used text attribute codes that are not supported by all terminals.
 const (
 	Border Attribute = iota + 51
 	Outline
@@ -99,6 +123,7 @@ const (
 	DisableUpperline
 )
 
+// ColorStringBuilder incrementally builds ANSI-colored strings.
 type ColorStringBuilder struct {
 	str string
 }