Allow index-files to specify file target list?

Instead of manually specifying `--include=src/file1.js`, `--include=src/file2.js`, `--include=src/file3.js`, etc, hundred/thousands of times for large projects, it would be really handy if I could just provide the list of files I want indexed.

Usage would look like:

```sh
codeql database index-files --language=javascript --file-list=targets.txt my_database
```

And `targets.txt` would look like:
```text
src/file1.js
src/file2.js
src/file3.js
```

(Obviously in a real world use-case, it would be a much longer list.)

Semi-related: #9.