dub should be more command-line oriented

[Luís Marques]

I often feel that dub relies too much on dub.[json|sdl] and too 
little on command-line switches. Here's the latest example:

1) Write code
2) Start refactoring code
3) Compile and run to check the partially implemented change
4) dub gives a warning; the warning is expected due to the 
incomplete change; I don't care about it right now, I just want 
to check that something related to that change works
5) Because of the warning, dub doesn't even finish building the 
project
6) I want to force dub to build it, but there's no switch for that
7) I have to go check the docs for what is the dub.json setting 
that allows warnings (it's "buildRequirements": ["allowWarnings"])
8) I change dub.json
9) I compile it again

universe 1:
10) I finish the refactoring
11) I forget to remove the new setting

universe 2:
10) I remove "allowWarnings" immediately
11) while trying to do another intermediate testing step, I get 
another warning
12) goto 7 or 8