Allow spaces in regexp repetition operators.

For instance, `x{n, m}`, `x{n ,m}` and `x{n  ,  m}` are all equivalent to `x{n,m}`. This is done for compatibility with YARA-X, but I've seen real-life regular expressions in that contain such spaces, for example:

```
/C:\\Users\\qqq\\.{30, 50}\.pdb/
```