Too strong constraint on metadata name

The commit https://github.com/peijunz/ipynb2pelican/commit/ec532ca9abf38dc09d40c2e6448c7d5f4bf3696b
has enabled `+` and also `*` as the start of metadata information. But its regex is too restrictive and fails on name of metadata with `*-_` characters like `metadata_suffix`.
```
+ metedata_suffix: This would cause regex to fail  
```