* Only run stack if a stack.yaml config is found
It is necessary to check for a stack.yaml file to distinguish between
cabal-only projects or stack projects (which are also cabal projects
since stack is built on top of cabal).
* Test that stack is called if stack.yaml exists
* Vim scripts shouldn't have hyphens
Especially not ones that will be autoloaded. You can't have a hyphen in
a function name, so autoloading functions based on filename will fail.
* Add g:haskell_stack_build_options, default: --fast
If we're going to use the --fast option, we may as well go the whole 9
yards and let the user configure the 'stack build' flags.
* Create documentation for stack-build options
