Add option presets

[jimporter]

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:

global_options([...], lang='c++', preset='debug')

executable(
    'exe',
    'src.cpp',
    compile_options=opts.preset([...], 'release')
)