Add vertical line for this moment

[hotoo]

When we use markline to make the future time line, may need a vertical line to point this moment, like:

Add now meta data for this feature, now support 3 values:

• smart, default, when has future event, show this moment vertical line.
• show, always show this moment vertical line.
• hide, always hide this moment vertical line.

for example:

# markline

- age: show
- now: smart

---

* 2999-01-01 the future