[Improvement] Tab: add line-width prop (#988)

docs/markdown/en-US/tab.md

@@ -141,6 +141,7 @@ In swipeable mode, you can switch tabs with swipe gestrue in the content
 | v-model | Index of active tab | `String` `Number` | `0` | - |
 | type | There are two style tabs, set this attribute to change tab style | `String` | `line` | `card` |
 | duration | Toggle tab's animation time | `Number` | `0.2` | - | - |
+| line-width | Width of tab line (px) | `Number` | Equal to current tab width | - |
 | swipe-threshold | Set swipe tabs threshold | `Number` | `4` | - | - |
 | sticky | Whether to use sticky mode | `Boolean` | `false` | - |
 | swipeable | Whether to switch tabs with swipe gestrue in the content | `Boolean` | `false` | - |

docs/markdown/zh-CN/tab.md

@@ -143,6 +143,7 @@ export default {
 | v-model | 当前激活的 tab | `String` `Number` | `0` | - |
 | type | Tab 样式类型 | `String` | `line` | `card` |
 | duration | 切换 tab 的动画时间 | `Number` | `0.2` | - |
+| line-width | 底部条宽度（px） | `Number` | 与当前 Tab 等宽 | - |
 | swipe-threshold | 滚动阀值，设置 Tab 超过多少个可滚动 | `Number` | `4` | - |
 | sticky | 是否使用粘性定位布局 | `Boolean` | `false` | - |
 | swipeable | 是否可以滑动内容切换 | `Boolean` | `false` | - |