Add --resolver param to stack build

[k-bx]

Stack comes with lts-2.9 resolver in stack.yaml. I know I already have a lot installed for lts-2.16 set of packages, and using this resolver would let me:

1. save disk space
2. save build time

Right now I had to edit stack.yaml, but what I'd like instead is to be able to do stack build --resolver lts-2.16.

As usual, if this seems like a good feature, I can put that on my queue and implement.

[snoyberg]

Did you try running that command? It should actually work exactly as you typed it :)

[snoyberg]

Sorry, it seems that --resolver lts-2.16 has to come before build, so stack --resolver lts-2.16 build. I'm not sure why that's the case, @chrisdone any thoughts?

[k-bx]

Oops, I only read stack build --help and didn't find resolver there.

[snoyberg]

I'll open a separate issue about allowing --resolver to be placed either before or after the command.