Update file parsing to handle lines with only whitespace.

[captainreality]

This commit updates file parsing so that lines that contain solely whitespace (as defined by by Unicode) are ignored. It's fairly common for people to accidentally create CODEOWNERS files with lines containing solely whitespace, and as it's semantically meaningless, it's better to ignore them then raise an error.

[captainreality]

Closed the PR because it was merged in #26.

[hmarr]

Thanks @captainreality, I've just pushed a fix for this in 06d1caa