Add example code for listing tags

[orirawlings]

Some questions about APIs came up for tag interaction in #430, this is meant to add some example code to demonstrate usage.

[mcuadros]

Can you link this example in the README file at the _example folder?

[mcuadros]

Also please take a look to the example, it doesn't exit with 0,

   --- FAIL: TestExamples/tags (1.47s)
    	common_test.go:119: error running cmd "exit status 1"

[orirawlings]

Whoops, didn't realize that I needed to run go test ./_examples --examples with the additional flag. Should be fixed now.