Support expands dot variables and add go mod file

[fifsky]

In unit tests varibales with "." in the name is allowed

	// it 'parses varibales with "." in the name' do
	// expect(env('FOO.BAR=foobar')).to eql('FOO.BAR' => 'foobar')
	parseAndCompare(t, "FOO.BAR=foobar", "FOO.BAR", "foobar")

So, expands should support varibales with "." in the name：

FOO.BAR=test
FOO2=${FOO.BAR}

And, I added the go mod file. Thanks!