ObsidianLoader fails when encountering template variables in the properties frontmatter of a file
Labels: name: area: doc loader, name: auto:bug

### System Info

langchain version 0.0.340
Python version: 3.11.5

### Who can help?

_No response_

### Information

- [ ] The official example notebooks/scripts
- [ ] My own modified scripts

### Related Components

- [ ] LLMs/Chat Models
- [ ] Embedding Models
- [ ] Prompts / Prompt Templates / Prompt Selectors
- [ ] Output Parsers
- [X] Document Loaders
- [ ] Vector Stores / Retrievers
- [ ] Memory
- [ ] Agents / Agent Executors
- [ ] Tools / Toolkits
- [ ] Chains
- [ ] Callbacks/Tracing
- [ ] Async

### Reproduction

1. Create an Obsidian template with a [template variable](https://help.obsidian.md/Plugins/Templates#Template+variables) in the [properties](https://help.obsidian.md/Editing+and+formatting/Properties#Property+format) section of the file.
2. Attempt to load a directory containing that template file using [ObsidianLoader](https://github.com/langchain-ai/langchain/blob/master/libs/langchain/langchain/document_loaders/obsidian.py).

```shell
$ echo -e "---\nyear: {{date:YYYY}}\n---" > vault/template.md
$ python
>>> from langchain.document_loaders.obsidian import ObsidianLoader
>>> loader = ObsidianLoader('vault')
>>> loader.load()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/user/.conda/envs/localai/lib/python3.11/site-packages/langchain/document_loaders/obsidian.py", line 115, in load
    front_matter = self._parse_front_matter(text)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.conda/envs/localai/lib/python3.11/site-packages/langchain/document_loaders/obsidian.py", line 48, in _parse_front_matter
    front_matter = yaml.safe_load(match.group(1))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.conda/envs/localai/lib/python3.11/site-packages/yaml/__init__.py", line 125, in safe_load
    return load(stream, SafeLoader)
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.conda/envs/localai/lib/python3.11/site-packages/yaml/__init__.py", line 81, in load
    return loader.get_single_data()
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.conda/envs/localai/lib/python3.11/site-packages/yaml/constructor.py", line 51, in get_single_data
    return self.construct_document(node)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.conda/envs/localai/lib/python3.11/site-packages/yaml/constructor.py", line 60, in construct_document
    for dummy in generator:
  File "/home/user/.conda/envs/localai/lib/python3.11/site-packages/yaml/constructor.py", line 413, in construct_yaml_map
    value = self.construct_mapping(node)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.conda/envs/localai/lib/python3.11/site-packages/yaml/constructor.py", line 218, in construct_mapping
    return super().construct_mapping(node, deep=deep)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/.conda/envs/localai/lib/python3.11/site-packages/yaml/constructor.py", line 141, in construct_mapping
    raise ConstructorError("while constructing a mapping", node.start_mark,
yaml.constructor.ConstructorError: while constructing a mapping
  in "<unicode string>", line 1, column 7:
    year: {{date:YYYY}}
          ^
found unhashable key
  in "<unicode string>", line 1, column 8:
    year: {{date:YYYY}}
```

### Expected behavior

[Template variables](https://help.obsidian.md/Plugins/Templates#Template+variables) are a feature in Obsidian and including them in the properties section of a file is perfectly valid, so [ObsidianLoader](https://github.com/langchain-ai/langchain/blob/master/libs/langchain/langchain/document_loaders/obsidian.py) should have no issue loading a directory that includes a file that has template variable in its properties.