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
```