fix(output): Re-work API to work with rustfmt

rustfmt will split long builers across lines, even when that breaks
logical grouping.

For example
```rust
assert_cli::Assert::command(&["ls", "foo-bar-foo"])
    .fails()
    .and()
    .stderr().contains("foo-bar-foo")
    .unwrap();
```
will be turned into
```rust
assert_cli::Assert::command(&["ls", "foo-bar-foo"])
    .fails()
    .and()
    .stderr()
    .contains("foo-bar-foo")
    .unwrap();
```
which obscures intent.

Normally, I don't like working around tools but this one seems
sufficient to do so.
```rust
assert_cli::Assert::command(&["ls", "foo-bar-foo"])
    .fails()
    .and()
    .stderr(assert_cli::Output::contains("foo-bar-foo"))
    .unwrap();
```

Pros
- More consistent with `with_env`
- Can add support for accepting arrays
- Still auto-complete / docs friendly
- Still expandable to additional assertions without much duplication or
  losing out on good error reporting

Cons
- More verbose if you don't `use assert_cli::{Assert, Environment, Output}`

Alternatives
- Accept distinct predicates
  - e.g. `.stderr(assert_cli::Is::text("foo-bar-foo"))`
  - e.g. `.stderr(assert_cli::Is::not("foo-bar-foo"))`
  - Strange `text` function
  - More structs to `use`
  - Less auto-complete / docs friendly (lacks contextual discovery or
    whatever the UX term is)

Fixes #70

BREAKING CHANGE: `.stdout().contains(text)` is now
`.stdout(assert_cli::Output::contains(text)`, etc.

Author: epage

README.md

@@ -37,20 +37,20 @@ fn main() {
     assert_cli::Assert::command(&["ls", "foo-bar-foo"])
         .fails()
         .and()
-        .stderr().contains("foo-bar-foo")
+        .stderr(assert_cli::Output::contains("foo-bar-foo"))
         .unwrap();
 }
 ```
 
 If you want to match the program's output _exactly_, you can use
-`stdout().is` (and shows the macro form of `command`):
+`Output::is` (and shows the macro form of `command`):
 
 ```rust,should_panic
 #[macro_use] extern crate assert_cli;
 
 fn main() {
     assert_cmd!(wc "README.md")
-        .stdout().is("1337 README.md")
+        .stdout(assert_cli::Output::is("1337 README.md"))
         .unwrap();
 }
 ```