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.

Originally based on #74