Markdown中的表格如何合并单元格以及如何换行

目前，markdown的语法不支持表格的单元格合并，只能使用 html 的表格源码来实现。举例如下：

<table>
    <tr>
        <td>A1</td>
        <td>B1</td>
        <td>C1</td>
        <td>D1</td>
    </tr>
    <tr>
        <td>A2</td>
        <td colspan="2">横跨两列</td>
        <td>D2</td>
    </tr>
    <tr>
        <td>A3</td>
        <td>B3</td>
        <td>C3</td>
        <td>D3</td>
    </tr>
</table>

<table>
    <tr>
        <td>A1</td>
        <td>B1</td>
        <td>C1</td>
        <td>D1</td>
    </tr>
    <tr>
        <td>A2</td>
        <td rowspan="2">横跨两行</td>
        <td>C2</td>
        <td>D2</td>
    </tr>
    <tr>
        <td>A3</td>
        <td>C3</td>
        <td>D3</td>
    </tr>
    <tr>
        <td>A4</td>
        <td>B4</td>
        <td>C4</td>
        <td>D4</td>
    </tr>
</table>

<table>
    <tr>
        <td>A1</td>
        <td>B1</td>
        <td>C1</td>
        <td>D1</td>
    </tr>
    <tr>
        <td>A2</td>
        <td colspan="2" rowspan="3">横跨三行两列</td>
        <td>D2</td>
    </tr>
    <tr>
        <td>A3</td>
        <td>D3</td>
    </tr>
    <tr>
        <td>A4</td>
        <td>D4</td>
    </tr>
    <tr>
        <td>A5</td>
        <td>B5</td>
        <td>C5</td>
        <td>D5</td>
    </tr>
</table>

重点来了，markdown 语法的表格中，单元格如何换行？只需使用 <br /> 即可，举例如下：

| 换行   | 未换行    |
|---       |---            |
| First <br /> Second <br /> Third  | First  Second  Third  |