Allow controlling card generation with a search string

Currently it is possible to generate a card conditionally depending on some fields being empty or not empty.

This approach is limited. For example I cannot automatically generate cards only if a field contains certain substring, which would be useful to automatically generate cloze deletion cards for certain words/symbols.
Also the current approach makes the card templates more complex and less readable, and sometimes, when the condition is complex, leads to code duplication in the card template.

To alleviate the problem I suggest adding a search string parameter per each card template. If the search string is not empty, it would limit card generation to the results of the search.