add support to multi line string

[drocha87]

This commit changed how the lexer behaves. Now in the first step of the lexer lex_file we just consumes the lines of the file and send it directly to lex_lines. I introduced a new main loop inside the lex_lines so we can walk line by line. I checked that the error reporting seems to be working fine.

[rexim]

@drocha87 looks goode to me! :+1: Thank you for your contribution!