Custom string preview

Would like to be able to transform the string before previewing (p) it, like piping to `jq` (or `fx` itself) to handle nested JSON, piping to a markdown rendered to handle markdown etc.

This is different from print (P) as the latter exits fx. This would stay instead. Process is:

1. Navigate to string
2. Press custom preview key
3. fx pipes the string to a program
4. fx displays the result from the program instead of a plain string