use the doublestar library to support pattern matching of files or
directories to ignore. This replaces (and deprecates) the previous
-skip flag which only supported file extensions.
Much of this is being tested with the existing file-based tests in
testdata/*, but this moves us toward a much more targetted and simpler
test structure.
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
The program now exits with a non-zero code when errors are encountered
while reading and/or writing files.
This now also introduces Go Modules for dependency management
