Fix handling source: docs/*.md pattern

[jzyinq]

In current solution such configuration:

source: docs/*.md

Would cause an error that Source file docs/*.md does not exist - obviously, this isn't a pointer to a single file but a wildcard suggesting that we're looking for set of files.

Additional check for "wildcardishness" for SourceFileMustExist validator should do the trick. I've added some test cases as a proof.