Change fname to shellescape when building command

Resolves #127

Long file names break on win32/gvim where 8.3 filenames no longer work
(with gVim 7.4 build).  vim-dispatch has been fixed to handle quoted long
file name parameters, this commit changes fname to shellescape.

Note - vim-dispatch must be updated before applying this patch.

Tested with vim-dispatch and vimproc on win32/gvim 7.4.

See: https://github.com/tpope/vim-dispatch/commit/acfd302ffab5b67dc7a3498c107c844f70d9ee5c