Let stack manage the Haskell Tools

[LeanderK]

with stack the GHC versions between the projects can differ, this can cause problems with the haskell tools. I therefore propose having the manual settings for cabal and let stack manage the tools similar to the way atom-haskell solves this problem (https://github.com/atom-haskell/haskell-ghc-mod/wiki/Using-with-stack). As a side benefit the installation and setup of the tools will be fully automatic. While not directly related, i am keeping this in mind while working on #298, so it should not require much work. While i would prefer letting stack manage the tools to be the standard, i think its a good idea to be able to disable that behaviour.

[carymrobbins]

Note that this can be easily implemented by relying on the stack build command, e.g.

% stack build ghc-mod
% stack build hlint

[carymrobbins]

I think these would be the requirements -

• When you open a project, if hlint and ghc-mod are not configured, prompt the user to see if he wants to have them auto-configured with stack. If the user says "no", somehow cache that response in the configs so we don't ask again.

• If the user says "yes", proceed to stack build hlint ghc-mod and pipe the output into a window. We already have a Haskell Tools Console, so maybe we can log Stack output there.

• Once the process completes, look up the built binaries for hlint and ghc-mod and configure the tools accordingly, notifying the user of success

• Obviously, if a failure occurs some point along the line, notify the user with a detailed error.