Plugin with evaluated call (#393)

* plugin trait

* impl of trait

* record and absolute path

* plugin example crate

* clippy error

* correcting cargo

* evaluated call for plugin

crates/nu-engine/src/call_ext.rs

@@ -1,10 +1,10 @@
 use nu_protocol::{
     ast::Call,
     engine::{EngineState, Stack},
-    ShellError,
+    FromValue, ShellError,
 };
 
-use crate::{eval_expression, FromValue};
+use crate::eval_expression;
 
 pub trait CallExt {
     fn get_flag<T: FromValue>(