This change fixes #801 as well as introduce a rather a lot of refactoring and new ParsedDocstring features.

• Parameters html are divided into .sig-param or .sig-symbol spans.
• When the function is long enough (88 chars excluding the def and ending colon, this is hard coded) an extra CSS class .long-signature is added to the parent function-signature.
• The first parameter 'cls' or 'self' of (class) methods is marked with the 'undocumented' CSS class, this way it's clearly not part of the API.
• Add some CSS to expand the signature of long functions when they have the focus.
• Refactor CSS rules for parameters tables to be more simpler and predictable with percentage values for smaller screens.
• Remove ValueFormatter classes since they were only a hack to show HTML tags inside Signature.__str__(), which we don't need anymore because we're computing the representation ourselve.
• Remove ParsedStanOnly class because we know have better alternatives.
• Introduce ParsedDocstring.to_text() -> str.
• ParsedDocstring.with_tag(Tag) -> ParsedDocstring.
• ParsedDocstring.combine(list[ParsedDocsrting]) -> ParsedDocstring.
• When a function has invalid parameters its rendered as (…) instead of confusing () - like it was a zero argument callable.

Examples:

• Not wrapped because overloaded function never get wrapped.

• Wrapped because the function length is over 88 chars

The way to get the old behaviour back is to use the following custom css:

.sig-param, .sig-symbol {
    display: initial !important;
    margin-left: 0 !important;
    padding-left: 0 !important;
}