Cleaned up code and user interface

Re: code: I added, updated and changed a lot of the comments and
switched to the full names of command line arguments whenever possible
(e.g., youtube-viewer -I becomes youtube-viewer --non-interactive).

Re: UI: "Client ready." now reads "ytsm ready.", many error messages and
warnings have been updated, use of color in the interface has been
tweaked slightly, and the old syntax error lines now mention ytsm's help
feature.