Add easy way to integrate XCodeGen to your own CLI project

[hakkurishian]

Motivated by the Use as dependency section in the XCodeGen readme, I thought there was an easy way to build my own command line tool that generates project.yml files according to my needs and then uses XCodeGen to actually generate projects. However, there does not seem to be an easy way to use that API. It would be great if a usage like this would be supported:

import XCodeGenKit

let project = Project(yml: pathToMyProjectYMLFile, options: nil)

project.generate() /// generates xcode project according to yml