Provide a stdin way of configuring files that need to be validated by GrumPHP

[veewee]

Q	A
Branch	master
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Documented?	yes
Fixed tickets	#394

This PR allows adding files to GrumPHP through stdin.
Example:

git diff | php ./vendor/bin/grumphp run
git diff --staged | php ./vendor/bin/grumphp run
git ls-files src | php ./vendor/bin/grumphp run
cat some-file-with-one-path-per-line.txt | php ./vendor/bin/grumphp run

This works for the commands: run, git:pre-commit, git:commit-msg