Add check for executables before installing

When setting up a new machine, it is possible to forget a few of the tools necessary to build and run certain Vim plugins (for example cmake for YouCompleteMe). It is particularly irritating when after cloning the repository I have to find out what binaries I need online and what script to execute.

To streamline this process, it would be better for Vim-Plug to gently notify the user that the required executables are not present on the system without pulling the repository at all. In the above example, we could then download cmake and run PlugInstall again to clone YouCompleteMe (which would then make a successful call to install.sh) and install successfully.