Obey excludes when finding targets

The TargetFinder should obey all excludes rules in the rubocop config, even if those file paths are passed into the CLI argument list.

Example with `db/**` on the excludes list:

```
# bad
rubocop db/schema.rb
Inspecting 1 file
#
# ... snip
1 file inspected, 2066 offences detected

# good!
rubocop db/schema.rb
Inspecting 0 files

0 files inspected, no offences detected
```

This will allow for better interactions with `guard`.