PlugSnapshot to use unexpanded plug home

/cc @andreicristianpetcu

plug#begin expands its path argument and converts it to the absolute
path by default. However, it makes sense to use the unexpanded form in
case of PlugSnapshot as described in
https://github.com/junegunn/vim-plug/issues/97#issuecomment-57421483

For example, for the following cases,

- call plug#begin('~/.vim/plugged')
- call plug#begin('$HOME/.vim/plugged')

PlugSnapshot will use the exact arguments, `~/.vim/plugged` or
`$HOME/.vim/plugged`, instead of the absolute paths such as
`/home/jg/.vim/plugged`.