doc: rounded avatars (#256)

Use weserv service to format avatars with circle masks. Make size configurable.

Tweak readme and adopters pages.

Updates #101

internal/cmd/docgen/main.go

@@ -10,8 +10,10 @@ import (
 	"fmt"
 	"io/ioutil"
 	"log"
+	"net/url"
 	"os"
 	"regexp"
+	"strconv"
 	"strings"
 	"text/template"
 
@@ -166,7 +168,26 @@ func snippet(filename, start, end string) (string, error) {
 }
 
 // avatar returns HTML for a Github user avatar.
-func avatar(owner string) (string, error) {
-	format := `<img src="https://github.com/%s.png?size=24" width="24" height="24" hspace="4" valign="middle" />`
-	return fmt.Sprintf(format, owner), nil
+func avatar(owner string, size int) (string, error) {
+	// Origin avatar URL from Github.
+	u := fmt.Sprintf("https://github.com/%s.png", owner)
+
+	// Use images.weserv.nl service to resize and apply circle mask.
+	v := url.Values{}
+	v.Set("url", u)
+	v.Set("w", strconv.Itoa(size))
+	v.Set("h", strconv.Itoa(size))
+	v.Set("fit", "cover")
+	v.Set("mask", "circle")
+	v.Set("maxage", "7d")
+
+	src := url.URL{
+		Scheme:   "https",
+		Host:     "images.weserv.nl",
+		RawQuery: v.Encode(),
+	}
+
+	// Build <img> tag.
+	format := `<img src="%s" width="%d" height="%d" hspace="4" valign="middle" />`
+	return fmt.Sprintf(format, src.String(), size, size), nil
 }