block math syntax and generated display math block should be inline-blocks.

[geyang]

Thanks for this great plug-in!

I noticed that the block math have to be on separate lines. Typographically LaTeX display math are inline-blocks and they can be typed inline. So the following syntax should be supported:

some text right here $$ \alpha $$ and some more inline math. 

In addition, the generated html should be:

<p>some text right here 
    <span display math here></span>
</p>

Right now the handling is incorrect. the result atm is:

<p> some text right here</p>
<span display math></span>
<p></p>