Add ignore configuration option for excluding globbed directories

[reid]

Feature request from @okuryu. See yui/yui3#778. For example:

{
    "basedir": ".",
    "glob": "**/tests/unit/*.html",
    "ignore": "node_modules/**"
}

This would exclude any files inside node_modules that otherwise match the glob. The option may be an array to specify multiple locations to ignore.