Only match includes at the beginning of the line

It seems that this is matching all links whether they start at the beginning of the line or not. Here's the regex in use:

```regexp
/\[(?=((?:[^\]]|\\.)*))\1\]\((?=((?:[^\)]|\\.)*))\2\)/gm
```

Seems like a simple fix would be:

```regexp
/^\[(?=((?:[^\]]|\\.)*))\1\]\((?=((?:[^\)]|\\.)*))\2\)/gm
```

This is related to #65. Another fix that would resolve both issues, and increase the performance of the plugin:

```regexp
/^\[(?=(import|include(?:[^\]]|\\.)*))\1\]\((?=((?:[^\)]|\\.)*))\2\)/gm
```

找出外掛程式用來比對 include 連結的正規表示式，然後使用位於行首和行中間的連結重現此問題。比較目前的行為與提議的表示式，並驗證比對僅限於行首，包括 #65 中描述的相關行為。

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