Add an example for KEY=value argument as asked in #79

Author: TeXitoi

examples/keyvalue.rs

@@ -0,0 +1,20 @@
+#[macro_use]
+extern crate structopt;
+
+use structopt::StructOpt;
+
+fn parse_key_val(s: &str) -> Result<(String, String), String> {
+    let pos = s.find('=').ok_or_else(|| format!("invalid KEY=value: no `=` found in `{}`", s))?;
+    Ok((s[..pos].into(), s[pos + 1..].into()))
+}
+
+#[derive(StructOpt, Debug)]
+struct Opt {
+    #[structopt(short = "D", parse(try_from_str = "parse_key_val"))]
+    defines: Vec<(String, String)>,
+}
+
+fn main() {
+    let opt = Opt::from_args();
+    println!("{:?}", opt);
+}