Support for a version flag

[jackson-theisen]

Currently, there is no good way to validate the installation of tfmask. Executing the binary by itself causes it to hang. It would be nice if it supported a -version flag to verify the binary was built/installed correctly.

Example:

wget --output-document="/usr/local/bin/tfmask" https://github.com/cloudposse/tfmask/releases/download/0.4.0/tfmask_linux_amd64 && chmod +x /usr/local/bin/tfmask
/usr/local/bin/tfmask -version
tfmask v0.4.0