Fix #97: Invoke ghc-mod with --silent flag

[jimenezrick]

This hides the warnings about if ghc-mod uses stack or cabal-install
which otherwise confuses the Vim plugin parsing the output.

[foreverbell]

Hello, @jimenezrick and @eagletmt , I am really doubtful whether using --silent flag is okay.

--silent flag gets into ghc-mod HEAD since this commit, https://github.com/DanielG/ghc-mod/blame/ad16b739ebab6e3ec3bc295619244fe42dcb2499/src/GHCMod/Options.hs#L77

Before ghc-mod version 5.5.0, invoking ghc-mod --silent causes ghc-mod to report ghc-mod: unknown command: --silent, I suggest using -s to get backward compatibility. Or we can follow the similar implementation from eagletmt/neco-ghc#65. What do you think?

UPD: I have raised a quick fix #100.

[jimenezrick]

Yeah, that makes sense @foreverbell, the -s quick fix looks good to me and maybe a similar approach to eagletmt/neco-ghc#65 would be better, maybe using --line-prefix=o,e as it's discussed in that other issue.