Enable querying non-Markdown files

[Kejikus]

Hello. I recently started using Obsidian as my file and note storage, and I have problem using query plugins such as this plugin or Dataview plugin. I want to query, sort and display lists and tables of any files in my vault, not just notes.

This PR changes the way file index is built by including all files in the vault and setting empty content and tags for non-Markdown files.

Note that these changes were not tested, but I'm pretty sure they will work.

I would also include some query parameter to enable or disable all-file search, but I do not know where is the code for parsing query parameters.