Added -X option - generate easily parseable file

Writes a portion of the default output into a separate file that is more easily parsed.

Sample output (btt.xvg):

DMI 8,16 310407 106729 2.908366049 0 182 1024 19504768 QSK 8,16 310407 167141.958551192 0 0 235753 DSK 8,16 106729 433247.436563633 0 0 33974 PLG 8,16 40824 382 0.008882101
UPG 8,16 1.993361748 1.866492147
ARQ 8,16 22.863331046

---
 btt/args.c        |   36 ++++++++++++++++++++++++-----
 btt/bt_timeline.c |    5 ++-
 btt/doc/btt.tex   |   56 +++++++++++++++++++++++++++++++++++++++++++--
 btt/globals.h     |    3 +-
 btt/output.c      |   64
+++++++++++++++++++++++++++++++++++++++++++++++++---
 5 files changed, 148 insertions(+), 16 deletions(-)