Use cmd for windows user instead of xterm
Check if the configuration directory exists

The previous file have a bug. `isdirectory('$APPDATA/Vifm')` always
return the same things because the variable is inside the quote.
In Windows I have found that Vifm, save the vimfiles in is directory
if a vifmrc is create.
Otherwhise It create this file into ~/.vifm/ if it exist.
Otherwhise It create inside the $APPDATA/Vifm/ (the directory create by
default).
Correct the typo
Document Vim plugin updates
Document additional repositories for Vim plugin

Resolves #35 issue at ksteen/vifm GitHub repository.
Merge branch 'minor-fixes-for-plugin-on-win'

* Make Vim plugin use cmd.exe as default shell on Windows.
* Fix "$APPDATA/Vifm" directory existence check for Vim plugin on
  Windows.

Thanks to Jonathan Da Silva.

Provide additional repository for Vim plugin (see the README for URL).