Add `serde` optional feature for cssparser-color

[Loirooriol]

#363 split the color parsing into a separate crate, which has things like

rust-cssparser/color/lib.rs

Line 19 in c3314d1

#[cfg(feature = "serde")]

However, its Cargo.toml doesn't define any serde feature.

@emilio

[chipnertkj]

@Loirooriol Hi there, coincidentally I needed the same thing done a while ago, here's my PR: #377

[emilio]

Fixed by #388, thanks @chipnertkj

[emilio]

Err, #378 (which is just a rebase of #377)