ls.go: Added basic functionality of long mode 

Added:
* Moved the path argument check into it's own function, named getPath().
* Added long mode, which currently only reports file size, date, and the filename.
* Replaced tabs with double spaces.

What's needed for complete functionality of long mode:
* Find a way to get a string of the fileMode bits.
* Find a way to obtain the user/group names for display.
* Shorten the date format
* If the file is a symbolic link, point to where the file really is (symlink -> location).
* Also colorize the file that the symlink is pointing at.
* Human readable mode, which translates the file size into a human readable format.