Update install script lnfile functionality

This allows me to specify the destination folder if the link name
doesn't need to be changed, i.e., if I want to install
vim/ftplugin/ruby.vim, I no longer need to specify the following line:
lnfile vim/ftplugin/ruby.vim ~/.vim/ftplugin/ruby

Instead, I can simply give the destination folder, just terminate it
with a slash character as follows:
lnfile vim/ftplugin/ruby.vim ~/.vim/ftplugin/