Use obsidian.json if vaultpath preference is empty

[cuddlecake]

• Makes the vaultpath preference optional
• Opens and loads vaults from obsidian.json file
• Shows "loading list" while vaults are loading (to wait for async file read)
• If vaultpath is present, it is preferred over obsidian.json

Closes #18

[cuddlecake]

You're welcome, and thanks for your work and for reviewing 😄