Add script to mix path with graph kmers

1. gpath_reader.c: gpath_reader_load() takes int flag to define behaviour when
   we hit kmers that are not in the graph (add, die with error, or ignore)

Perl scripts:
1. CortexPaths.pm: add function ctp_path_to_str()
2. scripts/cortex_mix_paths.pl interweave kmers with their paths:

    $ ./scripts/cortex_mix_paths.pl --kmer 9 genome.k9.ctx <(zcat reads.pe.two.ctp.gz)
    CAGTGGCCG 1 1 ...t...T ...t...T
    R 5 1 1 A
    CACTGATGC 1 1 .c...C.. .c...C..
    CACTGATGA 1 1 .c...C.. .c...C..
    R 5 1 1 G
    TCGGCATCA 1 1 ......G. ......G.
    CGGCATCAG 1 1 ...t...T ...t...T