Implement -check flag

When this flag is used:

* The program never modifies any files
* If all files in the pattern contain a license, the program exits with
a zero exit code
* If at least one file in the pattern requires modification to include
license text, the program prints such files to STDOUT and exits with
a non-zero exit code

README.md

@@ -18,6 +18,7 @@ to any file that already has one.
     -f custom license file (no default)
     -l license type: apache, bsd, mit (defaults to "apache")
     -y year (defaults to current year)
+    -check check only mode: verify presence of license headers and exit with non-zero code if missing
 
 The pattern argument can be provided multiple times, and may also refer
 to single files.