Feature: Add more Placeholder or Variable

[Sorrow-Scarlet]

Is your feature request related to a problem? Please describe.

Currently, I don't see any variable or placeholder when customizing my bonjourr.

Describe the solution you'd like

For example, When designing search bar, I'd like to do this:

Search Engine: Google
Placeholder Text: Search anything through $searchEngine

Or maybe in the Note section:

# Todo List
- [ ] Do sth...
- [ ] Do sth before $time+[00,00,00]
- [ ] ...

These placeholders are interesting and useful

Describe alternatives you've considered
Well... I can't find any alternative to placeholders or variable, maybe you could just use external url's json info? Or what else that can convey info quickly and simply.