fix a bug which caused a segfault when "MAX_VIDEOS" is reached

modify arg parsing: now just one iteration and one can do smth like this:
`caoydl -S word and another word without quotes`

`struct Item` is now `Item`, etc.

util.c now includes util.h + new utility function: `argvcat(char**)`

add additional commentaries
cosmetic changes
change default compiler to musl-gcc
create an uri scheme, "yt://", to prefix youtube IDs
fix channel video listing
Read from file in DEBUG mode

+ correct README and remove its "TODO" section
found a better way than to use `strlcat()` and `strlcpy()`
Update the README: dependencies, instructions to compile
Update Makefile to install man pages
doc, first attempt
Update usage in README
Huge refactor

- delete unused functions in util.[ch]
- added `emalloc()`
Make a separate header file for node processors
New feature: list playlists from channel
empty bug section
Update README + resolve a small bug
handle signature ciphered video stream URLs
Rewrite cli interface, uses arg.h by 20h

- add humantime() to util.c
Move what is subject to frequently change to "props.h"

- verify that provided IDs are proper ones
- get_pl_vids() now return all items from playlists the same way
  get_chan_urls() does