New Flag: -nodeps: skip target dependencies when executing target

[nlowe]

When you run mage with '-nodeps', mage will skip any Deps/SerialDeps
in the target being executed. This is useful when you want to run a
single target or need to run targets with dependencies locally but
without dependencies in CI for timing / visibility reasons.

Fixes #250

[nlowe]

Hmm, I'm not sure what that failure is on travis for Go: tip. I can't repro locally with 1.12 I get different failures on windows, but I can repro this with a 1.12 docker container. I'm going to open a new issue for this. Did go modules get flipped on by default?

#241 is also failing with the same error, I'm guessing this is unrelated to the changes in either PR.

[nlowe]

Ah, we're hitting #224.

[natefinch]

There's a lot of ways to do this if you really want to, but I don't think it's a good idea to add a new flag for it, since it should be rare.

You can either make the meat of the code a separate function and have one target with the deps and one without, or you can check an environment variable like this:

if os.Getenv("NODEPS") != "1" {
    mg.Deps(SomeDep)
}

But I don't feel like this is a common enough need to implement and maintain separately, especially since it's pretty easy to do yourself.