feat(tooltip): add support for rich text styles in tooltip.textStyle

[XHL-study]

Brief Information

This pull request is in the type of:

• [ ] bug fixing
• [x] new feature
• [ ] others

What does this PR do?

Now tooltip renderMode=richText, supports setting rich style

Fixed issues

Details

Before: What was the problem?

After: How does it behave after the fixing?

Document Info

One of the following should be checked.

• [ ] This PR doesn't relate to document changes
• [ ] The document should be updated later
• [x] The document changes have been made in apache/echarts-doc#432

Misc

ZRender Changes

• [ ] This PR depends on ZRender changes (ecomfe/zrender#xxx).

Related test cases or examples to use the new APIs

N.A.

Others

Merging options

• [ ] Please squash the commits into a single one when merging.

Other information

[echarts-bot[bot]]

Thanks for your contribution! The community will review it ASAP. In the meanwhile, please checkout the coding standard and Wiki about How to make a pull request.

Document changes are required in this PR. Please also make a PR to apache/echarts-doc for document changes and update the issue id in the PR description. When the doc PR is merged, the maintainers will remove the PR: awaiting doc label.

[XHL-study]

Please revert the changes of indent by using 4 spaces.

 I've fixed the indentation to 4 spaces. Please review again.

[jackz314]

@Ovilia could you take a look at the PR again? This would be a nice feature addition for our use case and it seems everything is ready.