Math Blocks should behave like code fences

[leipert]

Code fences can start right after a paragraph
```javascript
var x = y;
```
In my opinion math blocks should also work that way:
$$
x + y
$$
wuhu 

[runarberg]

Thank you

[geyang]

this is simply incorrect. The math blocks should not behave like code fences because

1. LaTeX does not enforce this, and a lot of users do not use $$ this way.
2. math blocks are inline-block elements instead of block elements. This should be respected by the markdown syntax as well.

I agree that it is visually appealing to have the display math blocks syntax like this, but again it is not standard and the current html generated is incorrect.

see #17