Add automatic download for haskell-language-server

[lukel97]

Closes #92 Wont work on Windows yet because lsp-mode upstream doesn't support decompressing gzips yet there. Also not sure what to do with lsp-haskell-server-path variables and friends. Probably needs a bit of a discussion

[lukel97]

Also need to figure out how to do versioning, e.g. what happens when a new version of hls is released. Probably best to just see what the other languages are doing. If they aren't doing any version checks then it may just be fine to tell users to rm -r ~/.emacs.d/.cache/lsp/haskell

[joelmccracken]

Do yall mind if I pick this up and continue this work? I'd like to start contributing to the haskell language server effort, and since I know emacs lisp farily well, this is an obvious place to start

[michaelpj]

By all means! Do have a look at what lsp-mode provides, they have some helpers for bits of this now.