Allow Micro to Open Directories

[progmem]

There is a request (#1023) to have Micro change its working directory when a folder is given as the first argument. The idea would be, since the filemanager plugin works off the current directory, this would allow someone using the plugin to immediately run tree and have the filebrowser view where they want it. While I do like the filemanager plugin, I do see this kind of change as being focused towards a specific plugin, and while the easy route out would be to have micro do os.Chdir in the presence of a single folder, this doesn't give us any behavior for multiple folders, and I think an approach that handles multiple folders would be more plugin-agnostic.

For some background, some editors like Sublime Text allow a user to pass multiple directories to it: sublime /path/a ./path/b. This would open up both directories in Sublime's file manager. To allow something similar, I propose allowing micro to open a directory as an empty buffer. Following the example for sublime, the command micro /path/a ./path/b would open two tabs: an empty buffer with /path/a, and an empty buffer with ./path/b. I'm assuming, at this point, that plugins like filemanager can be reworked so tree would use the current view's filename as the path to spawn the view from.

For some of this to work I had to make some modifies to how saveToFile works; part of the work that went into it duplicates some code from SaveAs, and I'm pretty certain this approach can be refactored quite a bit. I wouldn't necessarily say that this pull request is ready to be merged because of that, but I wanted to put this out to see if this kind of behavior is desired; at a very minimum, micro.go should be modified to check to see if the argument passed is a directory (when NewBufferFromFile errors out).

As part of this, I did notice a behavior: micro never prompts a user to overwrite a file if they start with an empty buffer and save it over an existing file. This adds a check for that.