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/microcosm-cc/bluemonday/.travis.yml

@@ -1,6 +1,5 @@
 language: go
 go:
-  - 1.1.x
   - 1.2.x
   - 1.3.x
   - 1.4.x
@@ -11,6 +10,7 @@ go:
   - 1.9.x
   - 1.10.x
   - 1.11.x
+  - 1.12.x
   - tip
 matrix:
   allow_failures: