Improve FileFilter on input files

1. Update file filter syntax: <into>:file.ctx:<from> where <into> and <from> can
both be lists, ranges or empty. Examples:
 * 0,2,6-9:in.ctx:0-5  {0,1,2,3,4,5} -> {0,2,6,7,8,9}
 * 0:in.ctx:0,2,3      {0,2,3} -> {0,0,0}
 * in.ctx:0,1,3        {0,1,3} -> {0,1,2}

2. loading_stat.c:
  a. rename stats.contigs_loaded -> stats.contigs_parsed
  b. remove num_files_loaded, ctx_files_loaded, num_of_colours_loaded
Clean up of FileFilter code

1. objbuf_macro.h:
  a. Fix BUG in _shift_left() function
  b. Add _shift_right() function

2. file_filter.c:
  a. use objbuf_macro.h instead of own buffer code
  b. use macros to access file_filter fields instead of being explicit
  c. add into_offset param to file_filter_set_cols(), to default if no
     `into filter' given.

3. ctx_join.c, ctx_pjoin.c: remove --flatten, --overlap options. They are not
   needed now that we have the more powerful input filters.

4. ctx_subgraph.c: fix bug where fromcol/intocol were mixed up when setting
   graph name