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/alecthomas/chroma/.travis.yml

@@ -4,7 +4,7 @@ go:
   - "1.13.x"
 script:
   - go test -v ./...
-  - curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | bash -s v1.22.2
+  - curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | bash -s v1.26.0
   - ./bin/golangci-lint run
   - git clean -fdx .
 after_success: