License header checker

[mbrukman]

It would be nice to have a utility that would run over a given set of files and output whether or not each of them have a valid header comment with a license in it.

This could be used in presubmit checks to ensure that no new files are added without such a license.

It could also be run from a unit test.

Caveats:

• need to handle years in the past
• need to handle whitespace changes, e.g., new lines or indentation changes
• need to handle minor changes around the license, such as flags to the #!/... line
• [maybe] avoid handling reflowed text, as that's a difficult problem

For an example, see Java Checkstyle.

[mbrukman]

https://github.com/google/addlicense appears to handle this use case already.