Added conversion support for markdown equations.

[egordm]

I have noticed when you load a block which contains inline equations, for example ($$A \leftarrow B$$), then it is rendered as a single character (⁍ / \u204d). This is mostly because the inline equation blocks are formatted the following way ["⁍", [["e","A \leftarrow B"]]] which is currently not interpreted correctly.

This PR adds correct conversion from and to markdown for inline equations. While markdown does not natively support inline equations, I decided to use the same block enclosing notion uses ($$).

Thanks in advance.

Example usage:

from notion.block import BulletedListBlock

newchild = page.children.add_new(BulletedListBlock, title="Some fancy equation: $$y = ax + b$$...")

[jamalex]

Thanks for the contribution! If you want to help prevent this from breaking down the road, you might also consider adding some pieces to the smoke_test.py scenarios (and perhaps your short example to the README).