Cygwin Support

Support for running OmniSharp though a cygwin vim.
* OmniSharp needs Window paths. Cygwin vim has unix-like paths. So, I call the buildin cygpath exe to convert them before sending them.
* However, cygwin shell treats the backslash in the Window path as an escape character, so the path is incorrect when OmniSharp gets it. So, I switched to not using shell to avoid this.