subscript by {} make a mess if one formula has multiple subscripts and latex {} pairs

[HowcanoeWang]

As in the previous image shows, here is the code for test:

This works:    
text text text $$\mathbf{x}_0, \mathbf{x}_1 = A \mathbf{x}_0, \mathbf{x}_2 = A \mathbf{x}_1$$,  text text -> $$x_1$$ <- text

However if want $$x_{1+k}$$ in previous sentenses which has two inline forumlar:     

text text text $$\mathbf{x}_0, \mathbf{x}_1 = A \mathbf{x}_0, \mathbf{x}_2 = A \mathbf{x}_1$$,  text text -> $$x_{1+k}$$ <- text

Seems don't work

[HowcanoeWang]

And also some interesting bugs, use `A_{B}` to replace `A_B` one by one:

original:

$$\mathbf{x}_0, \mathbf{x}_1 = A \mathbf{x}_0, \mathbf{x}_2 = A \mathbf{x}_1$$

replace 1 works

$$\mathbf{x}_{0}, \mathbf{x}_1 = A \mathbf{x}_0, \mathbf{x}_2 = A \mathbf{x}_1$$ 

replace 2 doesn't work

$$\mathbf{x}_{0}, \mathbf{x}_{1} = A \mathbf{x}_0, \mathbf{x}_2 = A \mathbf{x}_2$$ 

[obedm503]

This is very similar to #21. By the time the extension gets to process the html, showdown has already converted underscores to italics. Perhaps the long term solution is to handle conversion before showdown does anything. I'll have a go at it in a different branch.

[obedm503]

Begun conversion at feat-convert-to-lang. Help is welcome.

[VincentTam]

thanks for new feature :)