Added regex capture groups support for slug in targetPath and linkPattern

[lagthedark]

I added support for regex capture groups in MarkdownPaste.imageRules.

I needed it to use capture groups from the match regex in targetPath and linkPattern using $1, $2, etc.

"MarkdownPaste.imageRules": [
  {
    "match": "(.*)-example\\.md$",
    "targetPath": "${workspaceFolder}/images/$1/image.png",
    "linkPattern": "![${altText}](../images/$1/image.png)"
  }
]

I also updated the README.md to show this feature