Allow customizing the markdown parser (#2075)

* Allow customizing the markdown parser

For instance, code using Markdown might wish to create a markdown
parser that does not parse embedded HTML:
```py
def parser_factory():
    parser = MarkdownIt("gfm-like")
    parser.options["html"] = False
    return parser
```

* blacken

* Implement requested changes

* fix AttributeError

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Unreleased
 
+### Added
+- Added `parser_factory` argument to `Markdown` and `MarkdownViewer` constructors https://github.com/Textualize/textual/pull/2075
+
 ### Changed
 
 - Dropped "loading-indicator--dot" component style from LoadingIndicator https://github.com/Textualize/textual/pull/2050

src/textual/widgets/_markdown.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from pathlib import Path, PurePath
-from typing import Iterable
+from typing import Iterable, Callable
 
 from markdown_it import MarkdownIt
 from rich import box
@@ -541,6 +541,7 @@ class Markdown(Widget):
         name: str | None = None,
         id: str | None = None,
         classes: str | None = None,
+        parser_factory: Callable[[], MarkdownIt] | None = None,
     ):
         """A Markdown widget.
 
@@ -549,9 +550,11 @@ class Markdown(Widget):
             name: The name of the widget.
             id: The ID of the widget in the DOM.
             classes: The CSS classes of the widget.
+            parser_factory: A factory function to return a configured MarkdownIt instance. If `None`, a "gfm-like" parser is used.
         """
         super().__init__(name=name, id=id, classes=classes)
         self._markdown = markdown
+        self._parser_factory = parser_factory
 
     class TableOfContentsUpdated(Message, bubble=True):
         """The table of contents was updated."""
@@ -606,7 +609,11 @@ class Markdown(Widget):
         """
         output: list[MarkdownBlock] = []
         stack: list[MarkdownBlock] = []
-        parser = MarkdownIt("gfm-like")
+        parser = (
+            MarkdownIt("gfm-like")
+            if self._parser_factory is None
+            else self._parser_factory()
+        )
 
         content = Text()
         block_id: int = 0
@@ -831,6 +838,7 @@ class MarkdownViewer(VerticalScroll, can_focus=True, can_focus_children=True):
         name: str | None = None,
         id: str | None = None,
         classes: str | None = None,
+        parser_factory: Callable[[], MarkdownIt] | None = None,
     ):
         """Create a Markdown Viewer object.
 
@@ -840,10 +848,12 @@ class MarkdownViewer(VerticalScroll, can_focus=True, can_focus_children=True):
             name: The name of the widget.
             id: The ID of the widget in the DOM.
             classes: The CSS classes of the widget.
+            parser_factory: A factory function to return a configured MarkdownIt instance. If `None`, a "gfm-like" parser is used.
         """
         super().__init__(name=name, id=id, classes=classes)
         self.show_table_of_contents = show_table_of_contents
         self._markdown = markdown
+        self._parser_factory = parser_factory
 
     @property
     def document(self) -> Markdown:
@@ -882,7 +892,7 @@ class MarkdownViewer(VerticalScroll, can_focus=True, can_focus_children=True):
 
     def compose(self) -> ComposeResult:
         yield MarkdownTableOfContents()
-        yield Markdown()
+        yield Markdown(parser_factory=self._parser_factory)
 
     def on_markdown_table_of_contents_updated(
         self, message: Markdown.TableOfContentsUpdated