[功能] 是否能够支持提供默认的样式模板以及一些小建议

Executable	Version
node --version	8.12
npm --version	6.4.1
nodeppt -v	2.1.8
Windows 10	1903

After using the software carefully, I think it is best to display PPT in md, with the best support for UI and animations. However, there are several problems that I would like to summarize. If they can be fixed or supported, I believe it will be very helpful to promote nodePPT:

1. As the title says, I hope to provide a default style theme (similar to a theme template provided by PPT). For example, if no specific theme is specified, all pages use the same background and layout, and the animation style also has a default value. Now, it feels like leaving the template provided by the author, and it is basically impossible to continue making it.. In addition, a large amount of style code is copied and pasted, and regarding animations:

• If it is a PPT transition, it is possible to use the simple one that is currently given to all pages.
• If it is a specific element, such as a list, it is possible to give a simple style to all pages (for example, delay is set to 500ms by default).
• If the above is too troublesome, please let the <slide> page have a place where the default can be set globally..

1. The yaml syntax declaration in the header of the document. Can the standard yaml used in hexo/typora be used to encapsulate it with --- parms --? It is more compatible and general. Now, it is easy to be misinterpreted by writing directly.
2. The support for mermaid for flowcharts is great, but it seems that the version used is low, and the new syntax of new charts and old charts is not supported. I do not know whether it is possible to upgrade the version of mermaid.js, or provide a way to manually modify the version. Then there is a troublesome place that if the flowchart is long, the lower part can only be displayed completely by adjusting the CSS, and even cannot scroll down (it can be rendered normally in hexo).
3. Code is displayed here. It would be great if we could achieve simple control of step-by-step highlighting (other code segments are gray). This would be very helpful for demonstrating code, which is the effect achieved by gitpitch.
4. The note tag will indeed appear white or flowery in the narrator mode. I saw an issue mentioning this, but I don't know how to set the font color of the prompt? (I tried to modify the theme color in yaml without effect)
5. The ordered list in md has some layout problems. For example, if the overall is center, using an ordered list such as 1. xx 2. xx will cause the sequence numbers and content to be scattered, but the unordered list is normal. Other things I can add, in general, the functions of nodePPT are indeed very many, but it is still quite troublesome to use, and the invasion of original md is relatively large.