[rpmbuild] implement --task-file and --task-url parameters
Although this seems like I am adding possibly unnecessary features,
I am actually doing it to be able to simplify copr-rpmbuild
input and reduce the amount of sidesteps when obtainig task
defintion (hint: we will be able to get rid of the whole SCM
thingy while providing a convenient alternative, that will not
mess our code at all).
Also, `--task-file` and `--task-url` are useful for development,
please see the RFE for more information.