Detect markdown line width, resolve relative URLs (#332)

~~this is semi-blocked by https://github.com/charmbracelet/glamour/pull/96, but behaviour isn't really worse than the previous behaviour (most links work, some are still broken)~~

#### testcase for link resolver
```
tea pr 332
tea checkout 332 && make install && tea pr 332
```

- [rel](./332)
- [abs](/gitea/tea/pulls/332)
- [full](https://gitea.com/gitea/tea/pulls/332)

Co-authored-by: Norwin Roosen <git@nroo.de>
Co-authored-by: 6543 <6543@obermui.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/332
Reviewed-by: 6543 <6543@obermui.de>
Reviewed-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

vendor/github.com/charmbracelet/glamour/glamour.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/muesli/termenv"
 	"github.com/yuin/goldmark"
+	emoji "github.com/yuin/goldmark-emoji"
 	"github.com/yuin/goldmark/extension"
 	"github.com/yuin/goldmark/parser"
 	"github.com/yuin/goldmark/renderer"
@@ -135,20 +136,17 @@ func WithEnvironmentConfig() TermRendererOption {
 // standard style.
 func WithStylePath(stylePath string) TermRendererOption {
 	return func(tr *TermRenderer) error {
-		jsonBytes, err := ioutil.ReadFile(stylePath)
-		switch {
-		case err == nil:
-			return json.Unmarshal(jsonBytes, &tr.ansiOptions.Styles)
-		case os.IsNotExist(err):
-			styles, err := getDefaultStyle(stylePath)
+		styles, err := getDefaultStyle(stylePath)
+		if err != nil {
+			jsonBytes, err := ioutil.ReadFile(stylePath)
 			if err != nil {
 				return err
 			}
-			tr.ansiOptions.Styles = *styles
-			return nil
-		default:
-			return err
+
+			return json.Unmarshal(jsonBytes, &tr.ansiOptions.Styles)
 		}
+		tr.ansiOptions.Styles = *styles
+		return nil
 	}
 }
 
@@ -187,6 +185,14 @@ func WithWordWrap(wordWrap int) TermRendererOption {
 	}
 }
 
+// WithEmoji sets a TermRenderer's emoji rendering.
+func WithEmoji() TermRendererOption {
+	return func(tr *TermRenderer) error {
+		emoji.New().Extend(tr.md)
+		return nil
+	}
+}
+
 func (tr *TermRenderer) Read(b []byte) (int, error) {
 	return tr.renderBuf.Read(b)
 }