Cygwin compatibility (cygpath)

[ar-nelson]

Cygwin Vim currently breaks ghcmod-vim because there isn't a (recent) Cygwin version of Haskell, therefore Vim and ghc-mod use different path formats (Unix vs. Windows). This patch fixes those compatibility issues. Specifically:

• Unix/Windows paths are translated as needed, using vimproc and cygpath, but only when has("win32unix") is true.
• Reading ghc-mod output takes into account that the path format may be different from Vim's (includes backslashes in path regex).
• Carriage returns are removed from ghc-mod output.