mmd2tex math delimiters

Hi, I mentioned this before in the mmd support forum, but it was buried in another issue I was talking about (related to LaTeX math environment `$$\begin{align)...`). In the hindsight I should have separated the 2 issues. I hope you don't mind I kind of repeat it here:

Currently, in mmd2html and mmd2tex, this is how the math delimiter is translated:

| MMD Source | HTML Output | LaTeX Output |
| --- | --- | --- |
| `$...$` | `\(...\)` | `$...$` |
| `$$...$$` | `\[...\]` | `$$...$$` |
| `\\(...\\)` | `\(...\)` | `$...$` |
| `\\[...\\]` | `\[...\]` | `\[...\]` |

The proposal is simple:

| MMD Source | HTML Output | LaTeX Output |
| --- | --- | --- |
| `$...$` | `\(...\)` | `\(...\)` |
| `$$...$$` | `\[...\]` | `\[...\]` |
| `\\(...\\)` | `\(...\)` | `\(...\)` |
| `\\[...\\]` | `\[...\]` | `\[...\]` |

Reasons:
1. the `\(...\)` and `\[...\]` delimiters are preferred in LaTeX, and `$$...$$` might cause errors with some ams packages.
2. so that the HTML output and LaTeX output are identical as far as math delimiter is concerned