Fixed missing letter when no common path prefix.

When the files being reported had no common prefix,
`util.findCommonBase()` returned the first character of the first path,
which resulted in letters missing from any other path.

The bug can be reproduced by running the README’s example command,
but within src:

	cd src
	plato -d report *.js

This fixes the issue by allowing the prefix to be empty.
check for existence, fixes #6
Merge branch 'master' of https://github.com/campersander/plato
allow modal options (-h -v) to ignore -d
added lint reports on overview, new options, logger
adding reports for plato
deleting cruft and testing out exclusion regex
Merge branch 'help_version' of https://github.com/alunny/plato
added descriptions
plato report