False positive for paths starting with /

[malykhinvi]

I haven't had enough time to come up with a solution, but I prepared a PR with tests to reproduce the issue: #4

1. filepath is /foo-bar
2. substr is /foo
3. containsPath returns true, but looks like it should return false if partialMatch is not passed.

Looks like the problem is here:

contains-path/index.js

Lines 67 to 69 in f22e056

	if (ch === '/') {
	return true;
	}