More robust handling for "Symbols" group in index

[sandersk]

Add parameterized functionality to allow for "Symbols" group that contains non-number symbols only, "Symbols" group that contains numbers and other symbols, or separate "Numbers" and "Symbols" groups for numbers and non-number symbols, e.g.:

<div data-type="indexdiv">
    <h3>Numbers</h3>
    ...
</div>

<div data-type="indexdiv">
    <h3>Symbols</h3>
    ...
</div>