Allow optional space in `mf` helper

Updated `mf_extract` to be less sensitive with whitespace. Specifically, without this patch, helper blocks with a space before `mf` were not captured. 

Example:
```html
    <template>
      <h1>{{mf 'header1' 'My Header'}}</h1> <!-- this worked and continues to work -->
      <h1>{{ mf 'header2' 'My InvisibleHeader' }}</h1> <!-- previously not captured by mf_extract -->
    </template>