feat: load custom tints via files

[lrstanley]

Would be nice to have support for loading custom themes/tints via something like:

func LoadTintsFromPath(paths ...string) error {}

And:

func main() {
    _ = tint.LoadTintsFromPath("some/**/glob/*/pattern")
}

I imagine we can also possibly support multiple different file types? json, yaml, toml, etc?