[Feature Request]tree component Icon

[silverWolf818]

What problem does this feature solve?

�升级到iview3.0后，大部分的组件如�Select，Collapse，Cascader，Dropdown图片都有替换，但是tree组件的图片没有替换，导致UI不一致。

What does the proposed API look like?

希望修改一下icon或者提供一个API自行修改icon，保证项目一致性。

[iview-bot]

Translation of this issue:
What problem does this feature solve?
After upgrading to iview3.0, most of the components such as �Select, Collapse, Cascader, and Dropdown are replaced, but the tree component pictures are not replaced, causing UI inconsistencies.
What does the proposed API look like?
I hope to modify icon or provide a API to modify icon to ensure project consistency.