Feature request: add where_exp from Jekyll or something similar

[texastoland]

What

https://github.com/jekyll/jekyll/blob/3f3a283018a976da11a0bfcc13a20d43d37ee29f/lib/jekyll/filters.rb#L209-L221

Why

My use case at this moment is filtering tags containing a substring but I believe where_exp would be more generally useful than where_contains, where_not_contains, etc.