If directory is given as first argument, cd to it and open empty file instead of producing error

Description of the problem or steps to reproduce

Pass a directory to micro as the first argument, it will complain that you tried to open a directory.
Maybe this is just me, but my preferred behaviour would be for micro to use that directory as its working directory, and just open a blank file. So that I could then open files from that directory using Ctrl+O, or using the Filemanager plugin.

My workflow in Sublime and Atom is to open the project directory I'm going to be working in, and then open the files I want from the sidebar. Micro's filemanager plugin makes that almost possible, except that I have to change directory first and then open micro.

Start at micro's command-line argument handling for the first argument and trace how directory paths are opened. Run micro with a directory, then verify that it uses that directory as the working directory, opens an empty file, and allows files to be opened through Ctrl+O or the Filemanager plugin.