Support textual searching via regular expressions

[gibsramen]

Came up during discussion with @fedarko

Might be useful to allow the user to specify a regular expression for which to search the feature space. This should hopefully allow users to specify (almost?) any textual query they like even if it is not explicitly supported in Qurro.

Examples:

Given features k__Bacteria and k__Whackteria and k__Smackteria

• k__(Bac|Whack)teria should match k__Bacteria and k__Whackteria but not k__Smackteria
• k__.*teria should match all three
• k__[^h]*teria should match k__Bacteria and k__Smackteria but not k__Whackteria
• k__[\w]{5}teria should match k__Whackteria and k__Smackteria but not k__Bacteria

I think this could be done a couple ways - easiest might just be to add a dropdown option for regex filtering.