fix(stdin): Provide a Command wrapper

Many of our problems with `stdin` are derived from using extension
traits. By moving away from them, we fix the API problems and make it
easier to add other features like timeout (assert-rs#10) or signalling (assert-rs#84).

So the new philosphy if:
- Core functionality is provided by extension traits
- Provide a convinience API that makes `Command` friendlier. These do
  not need to be generalized.  Other abstractions can provide their own
  (like `duct`).

Fixes assert-rs#73

README.md

@@ -21,15 +21,10 @@ assert_cmd = "0.11"
 Here's a trivial example:
 
 ```rust,no_run
-extern crate assert_cmd;
+use assert_cmd::Command;
 
-use std::process::Command;
-use assert_cmd::prelude::*;
-
-Command::cargo_bin("bin_fixture")
-    .unwrap()
-    .assert()
-    .success();
+let mut cmd = Command::cargo_bin("bin_fixture").unwrap();
+cmd.assert().success();
 ```
 
 ## Relevant crates

src/cargo.rs

@@ -124,14 +124,24 @@ where
     fn cargo_bin<S: AsRef<str>>(name: S) -> Result<Self, CargoError>;
 }
 
+impl CommandCargoExt for crate::cmd::Command {
+    fn cargo_bin<S: AsRef<str>>(name: S) -> Result<Self, CargoError> {
+        crate::cmd::Command::cargo_bin(name)
+    }
+}
+
 impl CommandCargoExt for process::Command {
     fn cargo_bin<S: AsRef<str>>(name: S) -> Result<Self, CargoError> {
-        let path = cargo_bin(name);
-        if path.is_file() {
-            Ok(process::Command::new(path))
-        } else {
-            Err(CargoError::with_cause(NotFoundError { path }))
-        }
+        cargo_bin_cmd(name)
+    }
+}
+
+pub(crate) fn cargo_bin_cmd<S: AsRef<str>>(name: S) -> Result<process::Command, CargoError> {
+    let path = cargo_bin(name);
+    if path.is_file() {
+        Ok(process::Command::new(path))
+    } else {
+        Err(CargoError::with_cause(NotFoundError { path }))
     }
 }