feat(application): add new metadata to frontmatter (#9)

* feat(frontmatter): frontmatter method to add key, values

* build: add pysnooper to aid in debugging

* feat(application): add new frontmatter

* build: clean up dev container

* fix(notes): diff now pretty prints in a table

* docs(readme): update usage information

* docs(readme): fix markdown lists

src/obsidian_metadata/models/metadata.py

@@ -197,6 +197,40 @@ class Frontmatter:
 
         return dict_values_to_lists_strings(frontmatter, strip_null_values=True)
 
+    def add(self, key: str, value: str | list[str] = None) -> bool:
+        """Add a key and value to the frontmatter.
+
+        Args:
+            key (str): Key to add.
+            value (str, optional): Value to add.
+
+        Returns:
+            bool: True if the metadata was added
+        """
+        if value is None:
+            if key not in self.dict:
+                self.dict[key] = []
+                return True
+            return False
+
+        if key not in self.dict:
+            if isinstance(value, list):
+                self.dict[key] = value
+                return True
+
+            self.dict[key] = [value]
+            return True
+
+        if key in self.dict and value not in self.dict[key]:
+            if isinstance(value, list):
+                self.dict[key].extend(value)
+                return True
+
+            self.dict[key].append(value)
+            return True
+
+        return False
+
     def contains(self, key: str, value: str = None, is_regex: bool = False) -> bool:
         """Check if a key or value exists in the metadata.
 
@@ -314,6 +348,19 @@ class InlineMetadata:
         """
         return f"InlineMetadata(inline_metadata={self.dict})"
 
+    def add(self, key: str, value: str | list[str] = None) -> bool:
+        """Add a key and value to the frontmatter.
+
+        Args:
+            key (str): Key to add.
+            value (str, optional): Value to add.
+
+        Returns:
+            bool: True if the metadata was added
+        """
+        # TODO: implement adding to inline metadata which requires knowing where in the note the metadata is to be added.  In addition, unlike frontmatter, it is not possible to have multiple values for a key.
+        pass
+
     def _grab_inline_metadata(self, file_content: str) -> dict[str, list[str]]:
         """Grab inline metadata from a note.