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/go.mod

@@ -3,10 +3,11 @@ module github.com/charmbracelet/glamour
 go 1.13
 
 require (
-	github.com/alecthomas/chroma v0.7.3
-	github.com/microcosm-cc/bluemonday v1.0.2
-	github.com/muesli/reflow v0.1.0
-	github.com/muesli/termenv v0.6.0
+	github.com/alecthomas/chroma v0.8.1
+	github.com/microcosm-cc/bluemonday v1.0.4
+	github.com/muesli/reflow v0.2.0
+	github.com/muesli/termenv v0.7.4
 	github.com/olekukonko/tablewriter v0.0.4
-	github.com/yuin/goldmark v1.2.0
+	github.com/yuin/goldmark v1.3.1
+	github.com/yuin/goldmark-emoji v1.0.1
 )