Add StringFilterKind exhaustive filters

[s0ssh]

Previously, the steamworks::StringFilterKind enum lacked the exhaustive filter options exposed by the Steamworks API, only exposing k_ELobbyComparisonEqual and k_ELobbyComparisonNotEqual, quite confusingly, as StringFilterKind::Include and StringFilterKind::Exclude respectively. I have patched these into the enum.

...
match filter {
    StringFilterKind::EqualToOrLessThan=> sys::ELobbyComparison::k_ELobbyComparisonEqualToOrLessThan,
    StringFilterKind::LessThan=> sys::ELobbyComparison::k_ELobbyComparisonLessThan,
    StringFilterKind::Equal=> sys::ELobbyComparison::k_ELobbyComparisonEqual,
    StringFilterKind::GreaterThan=> sys::ELobbyComparison::k_ELobbyComparisonGreaterThan,
    StringFilterKind::EqualToOrGreaterThan=> sys::ELobbyComparison::k_ELobbyComparisonEqualToOrGreaterThan,
    StringFilterKind::NotEqual=> sys::ELobbyComparison::k_ELobbyComparisonNotEqual,
}
...

This PR resolves #218.