Make --build-directory and --install-directory required if the user asks for it

[clalancette]

After #13 , both of the --build-directory and --install-directory arguments are optional. However, we should add a way for the user to specify in their rosdoc2.yaml that a build is required in order to successfully document this particular package.xml. That flag could then be used by the rosdoc2 command-line to ensure that those paths are passed in the case that they are required. The buildfarm could also use that flag to determine whether or not it should bother building the package before trying to generate documentation for it.