Add a flag to allow empty files

[choldgraf]

A few times now we've run into issues w/ tests because we're running the validator on empty files:

https://travis-ci.org/bids-standard/bids-examples/builds/471063492#L2009

How do folks think about adding a flag to the validator like --allow-empty that would allow empty files to pass the validator, only for use with testing/dev.

[chrisgorgo]

The --ignoreNiftiHeaders flag was originally intended for testing purposes (https://github.com/bids-standard/bids-validator/blob/master/bin/bids-validator#L14). It might be worth replacing it with something more generic such as --ignoreEptyFiles.

Alternatively, you can pass a config file ignoring the 99 error.

[sappelhoff]

see also #651 and discussion therein

[choldgraf]

I'm equally +1 on:

1. Giving an --ignoreEmptyFiles flag and using this in the examples
2. Documenting (in the dev docs) the pattern that @sappelhoff described in the bids-examples PR

[jasmainak]

+1 also for me!

[chrisgorgo]

Are empty files allowed for some MEG datasets?
https://neurostars.org/t/error-using-bids-validator/3218/9

[sappelhoff]

@chrisfilo see #651