Add back book-keeping to outputs

[benkrikler]

FAST-RA1 (fast-carpenter's predecessor) had some nice book-keeping features that added meta-data into the dataframe as a comment-out header. It would be good to resurrect this in some form. Here's an example from the CHEP 2018 talk:

#% ===== HEADER (lines = 18)
# ancestors:
# - outputs/tbl_n.component.jet_pt.txt
# - outputs/tbl_cfg_component_phasespace_process.txt
# command_line:
# - t2df_combine_mc_components
# - outputs/backgrounds/
# filename: /home/ben/CMS/FAST/CMS_HEP_tutorial/outputs/tbl_n.proc
# project_dir: /home/ben/CMS/FAST/FAST-RA1/fast_ra1
# software:
#   ROOT_version: v6-08-06
#   pandas_version: 0.23.0
#   python_version: 2.7.13
#   rootpy_version: 1.0.1
#   yaml_version: '3.12'
# version: 0.1.0
# working_dir: /home/ben/CMS/FAST/180701_CMS_HEP_tutorial/
#
#% ===== dataframe (lines = 416)
component   dimu_mass            n          nvar
     data        -inf   993.000000  9.930000e+02