decode option event arg (#158)

Signed-off-by: Gregory Hill <gregorydhill@outlook.com>

src/metadata.rs

@@ -398,6 +398,7 @@ pub enum EventArg {
     Primitive(String),
     Vec(Box<EventArg>),
     Tuple(Vec<EventArg>),
+    Option(Box<EventArg>),
 }
 
 impl FromStr for EventArg {
@@ -413,6 +414,15 @@ impl FromStr for EventArg {
                     "Expected closing `>` for `Vec`",
                 ))
             }
+        } else if s.starts_with("Option<") {
+            if s.ends_with('>') {
+                Ok(EventArg::Option(Box::new(s[7..s.len() - 1].parse()?)))
+            } else {
+                Err(ConversionError::InvalidEventArg(
+                    s.to_string(),
+                    "Expected closing `>` for `Option`",
+                ))
+            }
         } else if s.starts_with('(') {
             if s.ends_with(')') {
                 let mut args = Vec::new();
@@ -439,6 +449,7 @@ impl EventArg {
         match self {
             EventArg::Primitive(p) => vec![p.clone()],
             EventArg::Vec(arg) => arg.primitives(),
+            EventArg::Option(arg) => arg.primitives(),
             EventArg::Tuple(args) => {
                 let mut primitives = Vec::new();
                 for arg in args {