[Feature]: Custom List Styles

There is currently a limitation that only three 'List Styles' are possible.
What I suggest is the ability to define custom styles in the settings.

Most simply, I guess this could be done by replacing the dropdown with two text fields. One for a prefix and another for a suffix.
These variables would then replace the switch statement in `generateFormattedIndexItem`; used to return the completed element `prefix + '[[${path}]]' + suffix`.

The above worked for me, though, as a quick hack. There should probably remain the current option of choosing from the preset styles plus 'other' in the dropdown. Only if 'other' were chosen would the user then be prompted to give a custom prefix and suffix.

Even better would be to give a whole string that somehow delimited where the data (`[[${path}]]`) would be inserted.