Let user add property to a frontmatter custom section

[Edo78]

To let user creating their metadata (maybe to use them with dataview or as they like) I can let create a frontmatter section and while rendering the template extracting that section and adding to a custom frontmatter property
Eg. the user made a template

---
date: <%= it.datetime.split(' ')[0] %>
year:: <%= new Date(Date.parse(it.datetime)).getYear() %>
---
lorem ipsum dolor sit amet

After rendering (presuming datetime equals to 2019-01-25 15:45:56 I'll have

---
date: '2019-01-25'
year: '2019'
---
lorem ipsum dolor sit amet

then easily use the body as usual while the frontmatter will be included in the original frontmatter
eg.

---
koreader-sync:
  uniqueId: 6df8a578d70600b70277474ee9062780
  [...]
  user_metadata:
    date: '2019-01-25'
    year: '2019'
---

and they can be accessed as koreader-sync.user_metadata.year