Lint .tsv

[kousu]

Fixes #57 (comment)

This can detect three problems:

• an incorrect number of fields
• a fully empty field (because nulls should be "-")
• trailing whitespace

It doesn't directly detect using spaces instead of tabs, but if you do that you'll probably trip the incorrect count or the trailing whitespace detector.

Example usage: After intentionally corrupting participants.tsv in each of the three ways:

$ .github/workflows/lint-tsv participants.tsv 
extraneous whitespace, line 2, column 4: '2019-02-12   '
errors in line 2: 
	'sub-amu01	M	28	2019-02-12   	amu	AMU - CEMEREM	Siemens	Verio	NeckMatrix	syngo_MR_B17	Virginie Callot'

empty field, line 3, column 7. Please use '-' for null values.
errors in line 3: 
	'sub-amu02	M	28	2019-02-13	amu	AMU - CEMEREM		Verio	NeckMatrix	syngo_MR_B17	Virginie Callot'

empty field, line 4, column 8. Please use '-' for null values.
errors in line 4: 
	'sub-amu03	F	28	2019-02-13	amu	AMU - CEMEREM	Siemens	    	NeckMatrix	syngo_MR_B17	Virginie Callot'

incorrect number of columns, line 6
errors in line 6: 
	'sub-amu05	F	39	2019-03-01	amu	AMU - CEMEREM	Siemens	Verio   syngo_MR_B17	Virginie Callot'

It's already added to .gitub/workflows/validator.yml to catch future glitches introduced by the Github web editor or whatever.

This is on top of #87 so please review + merge that first.

[jcohenadad]

didn't test it but awesome idea