Allow global exclusion patterns to be configured

[novemberborn]

As of #2103, AVA ignores certain directories by default, including any node_modules directories. We should make it possible to configure AVA with a custom set of exclusion patterns, overriding these defaults. For instance this can be used if you only want to ignore certain node_modules.

This can also be used to make the watcher more efficient. It disregards the exclusion patterns for files and sources when observing the file system, but should still respect the global exclusion patterns.