Importer feature updates for Roam, #236

[dropcase]

First attempt at adding features to the importer, related to Roam JSON import.

1. Added YAML frontmatter options:

• title option: adds the imported title with all characters
• created/edited timestamps option: checks the page- and block-level timestamps, uses the oldest for created and newest for edited

2. Extended the jsonToMarkdown function to add more values, could use some work (maybe pass an array instead?)
3. Changed the default replacement for illegal filename characters from '' to '_'

I tested with my Roam graph of 7100 notes, and didn't notice an increase in time to complete.

Still TODO: the timestamps are UTC, need some help sorting out time zone stuff.

Let me know if there are any issues or edits I need to make, thanks!