Metadata with comments

[2coffee2crab]

Closes #261

Some thoughts on the issue:
Several Markdown implementations allow for an optional feature called front matter, which allows the user to add metadata in YAML format at the start of a Markdown file, surrounded by three dashes.

markdown-oxide does support a small subset of it through its alias feature, which allows a user to add alternative names to the current file. The issue occurs when a comment is present in the front matter section. YAML comments are captured as headings, which is not desired.

Overview of the changes in this PR:

• src/vault/metadata.rs was extended to allow the MDMetadata struct to hold comments.
• Tests were added to ensure that regular comments (on a newline) and inline comments were correctly captured.
• The method MDFile::new() on src/vault/mod.rs was modified: the metadata is captured before the headings, which are filtered to ignore headings that are actually metadata.

Edit: accidentally included an unwanted commit, will fix.
Edit: fixed.