Bugfixes and New Paid Models: mfr-plus

Major changes:

• A new paid model, mfr-plus, has been added, which offers better recognition for multi-line formulas.
• When recognizing only English, CnOCR does not output Chinese.
• Bugs have been fixed.

主要变更：

• 加入新的付费模型：mfr-plus，对多行公式的识别效果更好。
• 在只识别英文时，CnOCR 不输出中文。
• 修复 bugs。