Allow adding css as basic string values for props not yet included

[stuartaccent]

this allows handling odd cases with more ease

eg:

Style{
    Selector: ".test",
    CustomProps: map[string]string{
        "--color":          "red",
        "background-color": "var(--color)",
    },
}

outputs:

.test{--color:red;background-color:var(--color);}

[stuartaccent]

https://github.com/AccentDesign/gcss/commit/f6b09de47361b51c5b03a33954b183b47fa968d4