Adds a way to set non-boolean build settings.

Hi!
As discussed on Twitter, I am here now with the pull request.
I added a way to set non-boolean build settings with Crafter. Although the way via 'options' is somehow redundant, I kept it because Crafter is a library and I don't want to force any user to change their configuration files.
Here is an example how to use it in the config:
set_build_settings ({
    :'OTHER_CFLAGS' => '-Wall'
})
I added a few checks too. In case someone wan't use 'options' or 'build_settings' in the config file.
Hope that is all ok with you.

Best,
Stephan