Remove trailing newline at the end of the string

I am using this crate, along with [`indent`](https://docs.rs/indent/latest/indent/), to generate code. For example:

```rust
let inner = indent::indent_by(4, indoc::indoc! {r#"
println!("hello world");
return true;
"#});

let outer = indoc::formatdoc! {r#"
fn do_things() -> bool {{
{inner}
}}
"#};
```

The result should be
```rust
fn do_things() -> bool {
println!("hello world");
return true;
}
```

but it actually produces
```rust
fn do_things() -> bool {
println!("hello world");
return true;

}

```

Notice the extra newlines after the strings created by the `indoc` macros. I could fix this by doing either
```rust
indoc::indoc! {r#"
stuff"#};
```
or
```rust
indoc::indoc! {r#"
stuff
"#}.trim_end().to_string();
```
but both of these options are ugly, and I am using this crate entirely to _not have ugly_. I think that this idea is appropriate considering this crate already removes the leading newline as well.