Add automatic management of markedly sessions on Windows

vim\markedly.vim: Use a similar cleanup mechanism for Windows, but this
time use a helper PowerShell script to kill off the markedly session.

vimrc: Provide a system independent g:path_separator variable and a
g:path_join() method which uses the separator.

vim\markedly_stop.ps1: A small PowerShell helper to find and kill the
relevant ruby markedly process.