Fix quotes on calling vim help on `:help`

cmd.exe doesn't understand single quotes ("'"), so use double quotes
instead, which should work for all shells on all systems (there are no
special symbols inside).
Reject :help command with args on 'novimhelp'

Don't accept arguments to the :help command when 'vimhelp' option
is off, because they are ignored anyway.  So this is just to make
mistake more explicit.
Pause on Windows on :help failed execution

Thanks to filterfalse.
Pause on Win, if executing of shell command failed

This was an old one, not sure why wasn't it done before.
Fix calling Vim on :help command with 'novimhelp'

In case when path to vifm's configuration directory contains spaces.

Thanks to filterfalse.
Merge branch 'fix-help-on-windows'