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/yuin/goldmark/ast/block.go

@@ -7,7 +7,7 @@ import (
 	textm "github.com/yuin/goldmark/text"
 )
 
-// A BaseBlock struct implements the Node interface.
+// A BaseBlock struct implements the Node interface partialliy.
 type BaseBlock struct {
 	BaseNode
 	blankPreviousLines bool
@@ -50,6 +50,8 @@ func (b *BaseBlock) SetLines(v *textm.Segments) {
 // A Document struct is a root node of Markdown text.
 type Document struct {
 	BaseBlock
+
+	meta map[string]interface{}
 }
 
 // KindDocument is a NodeKind of the Document node.
@@ -70,10 +72,29 @@ func (n *Document) Kind() NodeKind {
 	return KindDocument
 }
 
+// OwnerDocument implements Node.OwnerDocument
+func (n *Document) OwnerDocument() *Document {
+	return n
+}
+
+// Meta returns metadata of this document.
+func (n *Document) Meta() map[string]interface{} {
+	if n.meta == nil {
+		n.meta = map[string]interface{}{}
+	}
+	return n.meta
+}
+
+// SetMeta sets given metadata to this document.
+func (n *Document) SetMeta(meta map[string]interface{}) {
+	n.meta = meta
+}
+
 // NewDocument returns a new Document node.
 func NewDocument() *Document {
 	return &Document{
 		BaseBlock: BaseBlock{},
+		meta:      nil,
 	}
 }