Show issue reactions (#421)

```
$ tea issue 230

   #230 issue/pull details: show reactions (open)

  @6543 created 2020-10-22 16:39

  since reactions are utf8 now and most terminals too, we can display them nicely :)

  https://gitea.com/api/v1/repos/gitea/tea/issues/230/reactions

  --------

  1x 🎉  |  1x 👀  |  1x   |  1x 👍  |  1x 👎  |  1x 😆  |  1x 😕  |  1x ❤️
```

caveats:
- reactions are not returned as UTF8 (as was claimed in #230), so they need to be parsed. the library I use doesn't (and can't → ) support all reactions available in gitea
- currently only for issues, as reactions for comments mean an additional API request for each comment..

fixes #230

Co-authored-by: Norwin <git@nroo.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/421
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

vendor/github.com/enescakir/emoji/parser.go

@@ -0,0 +1,127 @@
+package emoji
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+	"unicode"
+)
+
+var (
+	flagRegex = regexp.MustCompile(`^:flag-([a-zA-Z]{2}):$`)
+)
+
+// Parse replaces emoji aliases (:pizza:) with unicode representation.
+func Parse(input string) string {
+	var matched strings.Builder
+	var output strings.Builder
+
+	for _, r := range input {
+		// when it's not `:`, it might be inner or outer of the emoji alias
+		if r != ':' {
+			// if matched is empty, it's the outer of the emoji alias
+			if matched.Len() == 0 {
+				output.WriteRune(r)
+				continue
+			}
+
+			matched.WriteRune(r)
+
+			// if it's space, the alias's not valid.
+			// reset matched for breaking the emoji alias
+			if unicode.IsSpace(r) {
+				output.WriteString(matched.String())
+				matched.Reset()
+			}
+			continue
+		}
+
+		// r is `:` now
+		// if matched is empty, it's the beginning of the emoji alias
+		if matched.Len() == 0 {
+			matched.WriteRune(r)
+			continue
+		}
+
+		// it's the end of the emoji alias
+		match := matched.String()
+		alias := match + ":"
+
+		// check for emoji alias
+		if code, ok := Find(alias); ok {
+			output.WriteString(code)
+			matched.Reset()
+			continue
+		}
+
+		// not found any emoji
+		output.WriteString(match)
+		// it might be the beginning of the another emoji alias
+		matched.Reset()
+		matched.WriteRune(r)
+
+	}
+
+	// if matched not empty, add it to output
+	if matched.Len() != 0 {
+		output.WriteString(matched.String())
+		matched.Reset()
+	}
+
+	return output.String()
+}
+
+// Map returns the emojis map.
+// Key is the alias of the emoji.
+// Value is the code of the emoji.
+func Map() map[string]string {
+	return emojiMap
+}
+
+// AppendAlias adds new emoji pair to the emojis map.
+func AppendAlias(alias, code string) error {
+	if c, ok := emojiMap[alias]; ok {
+		return fmt.Errorf("emoji already exist: %q => %+q", alias, c)
+	}
+
+	for _, r := range alias {
+		if unicode.IsSpace(r) {
+			return fmt.Errorf("emoji alias is not valid: %q", alias)
+		}
+	}
+
+	emojiMap[alias] = code
+
+	return nil
+}
+
+// Exist checks existence of the emoji by alias.
+func Exist(alias string) bool {
+	_, ok := Find(alias)
+
+	return ok
+}
+
+// Find returns the emoji code by alias.
+func Find(alias string) (string, bool) {
+	if code, ok := emojiMap[alias]; ok {
+		return code, true
+	}
+
+	if flag := checkFlag(alias); len(flag) > 0 {
+		return flag, true
+	}
+
+	return "", false
+}
+
+// checkFlag finds flag emoji for `flag-[CODE]` pattern
+func checkFlag(alias string) string {
+	if matches := flagRegex.FindStringSubmatch(alias); len(matches) == 2 {
+		flag, _ := CountryFlag(matches[1])
+
+		return flag.String()
+	}
+
+	return ""
+}