added capability of adding style/attribute to the title slide

For example, users can add the following to the header of the org file:
```
#+REVEAL_TITLE_ATTR: data-background=wordle.jpg
#+REVEAL_TITLE_ATTR: style="background-color: rgba(1, 1, 1, 0.6)"
```
This commit also includes @krisajenkins's change and some minor formattings.