support single file arguments to bin/precompile

I've added support for outputting the compiled code for a single file if one is passed as an argument to bin/precompile.

The code for loading the template at runtime isn't outputted as it is when you pass a dir.

This won't change anything for existing users that are passing dir arguments in. Previously passing a file not a dir in would have resulted in an error.

I'm going to write a basic syntastic plugin for nunjucks and this makes that easy - i'm more interested in the error output than compiled code - but maybe there's also some other use for people that want to assemble their precompiled templates themselves rather than the use the emitted code.