Feature request: --exclude flag

[weierophinney]

We often have test assets such as test configuration files that do not and should not have license docblocks. However, because they are PHP files, they are scanned and docheader correctly points out that they are missing license headers.

As such, I request an --exclude flag for the binary, and/or an --exclude-dir flag. The flag(s) should allow multiple invocations:

# Multiple --exclude flags:
$ docheader check test --exclude test/TestAsset/config.php --exclude test/Foo/TestAsset/config.php
# Multiple --exclude-dir flags:
$ docheader check src test --exclude-dir test/TestAsset --exclude-dir test/Foo/TestAsset

Any items matching the flag(s) would then be excluded when performing checks.

[malukenho]

Handled on #14