Add support of custom command to open ranger

[srghma]

I want to modify command, that open ranger to open it with custom SHELL (to load .zshrc before ranger for aliases) and custom theme (because default theme look ugly in my vim) like this
SHELL=/home/bjorn/.config/ranger/rshell ranger --cmd "set colorscheme snow"

[srghma]

Had switched to https://github.com/bramblex/ranger.vim/blob/master/plugin/ranger.vim