Introduce a flag to prefer `//` over `/*...*/`

It would be great to have a non-multiline comment support enabled with a new flag. Multiline comments `/*\n*\n*\n*/` takes more lines and makes it harder to read for me. Here is a simple proposal:

```
$ touch foo.c
$ addlicense -s=only -c Furkan .
```

Actual: (Multiline comment)
```
/*
* Copyright 2022 Furkan
* SPDX-License-Identifier: Apache-2.0
*/
```

New flag way: (comment)
```
// Copyright 2022 Furkan
// SPDX-License-Identifier: Apache-2.0
```

Start at the addlicense CLI and trace flag parsing to the code that renders license headers. Compare the existing multiline output with the requested // form and determine how the new flag should select it. Done means the flag produces the shown single-line comment format for foo.c while the current default remains unchanged.

Written by the indexing model from the issue text.