addurls: add detection (filename extension) and support for .tsv files

[yarikoptic]

.tsv is dominant file type (over .csv) in neuroimaging.

ATM addurls happily assumes .tsv to be a .csv clamping all fields into one, and even suggestions do not appear and with all the generators I end up with

$> datalad --dbg addurls -d . --message "Just a first few 'fast'" --fast datastructure_manifest.tsv '{associated_file}' '{filename}' --dry-run
addurls(error): /home/yoh/proj/abcd/dataset-try1 (dataset) [Unknown placeholder in URL: 'associated_file' [addurls.py:extract:480]]
Traceback (most recent call last):
  File "/home/yoh/proj/datalad/datalad-master/venvs/dev3/bin/datalad", line 11, in <module>
    load_entry_point('datalad', 'console_scripts', 'datalad')()
  File "/home/yoh/proj/datalad/datalad-master/datalad/cmdline/main.py", line 521, in main
    ret = cmdlineargs.func(cmdlineargs)
  File "/home/yoh/proj/datalad/datalad-master/datalad/interface/base.py", line 786, in call_from_parser
    ret = list(ret)
  File "/home/yoh/proj/datalad/datalad-master/datalad/interface/utils.py", line 469, in generator_func
    msg="Command did not complete successfully")
datalad.support.exceptions.IncompleteResultsError: Command did not complete successfully. 1 failed:
[{'action': 'addurls',
  'message': "Unknown placeholder in URL: 'associated_file' "
             '[addurls.py:extract:480]',
  'path': '/home/yoh/proj/abcd/dataset-try1',
  'status': 'error',
  'type': 'dataset'}]

> /home/yoh/proj/datalad/datalad-master/datalad/interface/utils.py(469)generator_func()
-> msg="Command did not complete successfully")
(Pdb) 

even without ability to see what placeholders it had in mind (nothing to suggest even remotely close within a set of 1 'submission_id\tdataset_id\tsubmission_id\tmanifest_name\tmanifest_file_name\tassociated_file\tfilename'