Support -version argument

[atz]

Since golicense is distributed in binary form, it would help debugging and differentiation for it to respond to -version in the conventional way.

While go version -m -v /path/to/golicense can provide some detail, this is not necessarily the same thing as a known SemVer identifier. Example:

go version -m -v `which golicense`
/usr/local/bin/golicense: go1.13.4
	path	github.com/mitchellh/golicense
	mod	github.com/mitchellh/golicense	(devel)	
	dep	github.com/360EntSecGroup-Skylar/excelize	v1.4.0	h1:43rak9uafmwSJpXfFO1heKQph8tP3nlfWJWFQQtW1R0=
	...

This is particularly telling because no devel git-ref currently exists in the project.