msgviewer: open http links from messages

Parse http links from a message and display them as completions in the
:open-link command.

Add the following binds to the [view] section in your binds.conf:
<C-l> = :open-link <space>

Parsing can be disabled in aerc.conf by setting parse-http-links to
false in the viewer section.

Thanks to Moritz for the help with the regular expression.

Signed-off-by: Koni Marti <koni.marti@gmail.com>
Reviewed-by: Moritz Poldrack <git@moritz.sh>
Acked-by: Robin Jarry <robin@jarry.cc>

lib/parse/hyperlinks.go

@@ -0,0 +1,44 @@
+package parse
+
+import (
+	"bufio"
+	"bytes"
+	"io"
+	"regexp"
+	"strings"
+)
+
+var submatch = `(https?:\/\/[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,10}\b(?:[-a-zA-Z0-9()@:%_\+.~#?&\/=]*))`
+var httpRe = regexp.MustCompile("\"" + submatch + "\"" + "|" + "\\(" + submatch + "\\)" + "|" + "<" + submatch + ">" + "|" + submatch)
+
+// HttpLinks searches a reader for a http link and returns a copy of the
+// reader and a slice with links.
+func HttpLinks(r io.Reader) (io.Reader, []string) {
+	var buf bytes.Buffer
+	tr := io.TeeReader(r, &buf)
+
+	scanner := bufio.NewScanner(tr)
+	linkMap := make(map[string]struct{})
+	for scanner.Scan() {
+		line := scanner.Text()
+		if !strings.Contains(line, "http") {
+			continue
+		}
+		for _, word := range strings.Fields(line) {
+			if links := httpRe.FindStringSubmatch(word); len(links) > 0 {
+				for _, l := range links[1:] {
+					if l != "" {
+						linkMap[strings.TrimSpace(l)] = struct{}{}
+					}
+				}
+			}
+		}
+	}
+
+	results := []string{}
+	for link, _ := range linkMap {
+		results = append(results, link)
+	}
+
+	return &buf, results
+}