Add support for buildFlag iOS args

[blackholegalaxy]

@Almouro Hi there!

Do you think it's possible to add support, on ios args, for buildFlag. Indeed it can be useful to push buildFlag to cordova compile command sometimes.

for example:

cordova compile ios --buildFlag='-UseModernBuildSystem=0'

buildFlag can be multiple and have then to be written:

cordova compile ios --buildFlag='-UseModernBuildSystem=0' --buildFlag='-anotherFlag=value'

PR opened, needs review:

#38

[jpgilchrist]

I think this is pretty critical given the release of xcode10... see related issues with cordova here: apache/cordova-ios#412

[blackholegalaxy]

PR opened, review welcome as Ruby is not quite my everyday business