Add option presets

It might be useful to allow `build.bfg` authors to add "option presets" to their config, allowing easy ways to set up build variants (debug, release, etc). What would this look like though? One option would be something like:

```python
global_options([...], lang='c++', preset='debug')

executable(
    'exe',
    'src.cpp',
    compile_options=opts.preset([...], 'release')
)
```