add interpolation variable matcher

here's something I'd love to use in devise-i18n 

based on something similar I made for loomio.
Please feel free to get me to rework anything that is unclear/ verbose 
this is an opinion which suits my context, where it's most important that there is either no variables in particular locale or exactly the right ones. 

open to discussing this/ changing.
as it is the `locale_vars` is an array of all the keys in a string. which could mean a case like this would pass.
```yaml
en:
  joined: #{user} joined the group

fr: 
  joined: joined the group
```