Allow to override default ignore with negative patterns

[pvdlg]

Fix #65

In lintFiles instead of passing the ignores to globby ignores options, this PR invert the ignores patterns and pass them to globby within the pattern array.
The ignores pattern from the ignores options are concatenated to the default one so they can override them.

We have to invert the ignores patterns and pass them to the pattern parameter because node glob doesn't handle opposed patterns in the ignore options. In other words passing ['dir/**', '!dir/**'] or ['dir/**'] in the ignore options exclude the files in dir in both case. But passing ['!dir/**', 'dir/**'] in the pattern argument does include the files in dir.

[pvdlg]

This solution might be a good candidate for globby. Not sure where is the best place.

[sindresorhus]

Yes, this should preferably be fixed in globby so all projects using it could benefit.

[sindresorhus]

@pvdlg What should we do about this one?

[pvdlg]

We can close this PR. It should be done in globby. I'll get back to it at some point unless someone want to give it a shot.