feature/char_string - printf handling `c`, `s` and `%`

[fernandogmo]

• Prototype: int _printf(const char *format, ...);
• Returns: the number of characters printed (excluding the null byte used to end output to strings)
• write output to stdout, the standard output stream
• format is a character string. The format string is composed of zero or more directives. See man 3 printf for more detail. You need to handle the following conversion specifiers:
  c
s
%
• You don’t have to reproduce the buffer handling of the C library printf function
• You don’t have to handle the flag characters
• You don’t have to handle field width
• You don’t have to handle precision
• You don’t have to handle the length modifiers