added support for richtext

[FelixMook]

with this support it is possible to use richtext from exceljs

[pofider]

Thank you for the PR!
I see the benefit. Multiple text parts with different styles in a single cell aren't supported now.

This is quite a fast solution for the problem, but I'm not sure how I like how the input HTML would look like then.
This is a bit ugly mix.

<td data-cell-type="rixtext">
{
  'richText': [
    {'font': {'size': 12,'color': {'theme': 0},'name': 'Calibri','family': 2,'scheme': 'minor'},'text': 'This is '}, 'format'}
  ]
}
</td>

I think I would rather prefer if we support multiple span elements inside a single cell like this.

<td>
   <span style='color: red'>I am red</span>
   <span style='color: blue'>I am blue</span>
</td>

What you think about it?

[FelixMook]

This solution is the cleaner way, but I think it will be hard to convert the css-style in the format for the richtext.
We could also think about a custom element like this:

<td>
   <span richtext-style=" {'font': {'size': 12,'color': {'theme': 0},'name': 'Calibri','family': 2,'scheme': 'minor'} ">This is the text</span>
</td>