Ignore node_modules in check-conflicts

[imliam]

I believe most PHP webapp monorepos are paired with a node_modules directory, so having this included in the check-conflicts command can significantly slow it down. As there's no way to pass through directories to exclude as options, I feel it's sensible to include as a default behaviour.

Here's the results on a pretty average app with/without this change:

With node_modules:

[OK] No conflicts found in 25686 files
7.5s total time taken

Without node_modules:

[OK] No conflicts found in 8818 files
1.4s total time taken

[samsonasik]

Looks good, thank you @imliam

[TomasVotruba]

Good point 👍 thanks for the PR