feat: add lazy static

[pleshevskiy]

I've added lazy static implementation for variables. Also added &'static str implementation. This type can be used by default

config! {
    static STATIC_STR => "test",
    static STATIC_STRING: String => "test",
    static STATIC_I8: i8 => 1,
    static STATIC_I16: i16 => 1,
    static STATIC_I32: i32 => 1,
    static STATIC_I64: i64 => 1,
    static STATIC_I128: i128 => 1,
    static STATIC_ISIZE: isize => 1,
    static STATIC_U8: u8 => 1,
    static STATIC_U16: u16 => 1,
    static STATIC_U32: u32 => 1,
    static STATIC_U64: u64 => 1,
    static STATIC_U128: u128 => 1,
    static STATIC_USIZE: usize => 1,
    static STATIC_F32: f32 => 1,
    static STATIC_F64: f64 => 1,
    static STATIC_CONCAT_VARIABLE < (
        "static ",
        STATIC_CONCAT_PART => "part",
    ),
}

Closes #13