Add `webvtt_lex_newline` func to read newlines in reentrant manner

Used by `T_CUEREAD` state and `read_cuetext()` routine in order to
avoid needing to decrement `self->line` (which is potentially not
incremented at all)
Added unit-tests for lex_newline()
Wrapped #include <libwebvtt/lexer.cpp> in extern "C" block
Fix Makefile missing separator error.
Merge branch 'lex-newline' of git://github.com/rillian/webvtt into lex-newline-ril
Include <libwebvtt/lexer.c> after C++ headers in lexer_unittest.cpp
Merge pull request #297 from caitp/webvtt-lex-newline

Add `webvtt_lex_newline` func to read newlines in reentrant manner