Add support for :: inside optional_derive

[igorzas]

The following has build failure

[derive(OptionalStruct)]

[optional_derive(Debug, Clone, PartialEq, serde::Deserialize)]

struct TestConfig { delay: Option, path: String, percentage: f32, }

error: proc-macro derive panicked --> src/main.rs:60:10	60	#[derive(OptionalStruct)]	^^^^^^^^^^^^^^

= help: message: called Result::unwrap() on an Err value: "failed to parse derive input: \"#[optional_derive(Debug, Clone, PartialEq, serde :: Deserialize)] struct\nTestConfig { delay : Option < u32 >, path : String, percentage : f32, }\""

While the following is build correctly (however would not work with serde serialization)

[derive(OptionalStruct)]

[optional_derive(Debug, Clone, PartialEq, Deserialize)]

struct TestConfig { delay: Option, path: String, percentage: f32, }

[lesurp]

Seems to be fixed by #15 (see test from c4223de). Now have issues for already existing derive directives...